feature #698 Update project to Symfony 4 beta4 (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #698).

Discussion
----------

Update project to Symfony 4 beta4

Changelog:

```
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 6 installs, 43 updates, 0 removals
  - Updating symfony/flex (v1.0.39 => v1.0.41)
  - Installing ocramius/package-versions (1.1.3)
  - Updating symfony/asset (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/cache (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/expression-language (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/inflector (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/property-access (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/options-resolver (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/intl (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/event-dispatcher (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/form (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/http-foundation (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/debug (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/http-kernel (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/filesystem (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/config (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/dependency-injection (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/monolog-bridge (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/monolog-bundle (v3.1.1 => v3.1.2)
  - Updating symfony/routing (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/finder (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/framework-bundle (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/yaml (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/console (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Installing zendframework/zend-eventmanager (3.2.0)
  - Installing zendframework/zend-code (3.3.0)
  - Installing ocramius/proxy-manager (2.1.1)
  - Installing doctrine/migrations (v1.6.0)
  - Updating symfony/doctrine-bridge (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Installing doctrine/doctrine-migrations-bundle (v1.3.1)
  - Updating symfony/security (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/security-bundle (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/translation (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/validator (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/stopwatch (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/process (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating friendsofphp/php-cs-fixer (v2.8.0 => v2.8.1)
  - Updating symfony/dom-crawler (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/browser-kit (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/css-selector (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/var-dumper (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/twig-bridge (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/debug-bundle (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/dotenv (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/phpunit-bridge (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/web-server-bundle (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/twig-bundle (v4.0.0-BETA3 => v4.0.0-BETA4)
  - Updating symfony/web-profiler-bundle (v4.0.0-BETA3 => v4.0.0-BETA4)
Writing lock file
Generating autoload files
Symfony operations: 1 recipe (4cbbdec1ecf0bb3138e2dae41ebc487d)
  - Configuring doctrine/doctrine-migrations-bundle (1.2): From github.com/symfony/recipes:master
ocramius/package-versions:  Generating version class...
ocramius/package-versions: ...done generating version class
```

Anybody knows what's going on with this unexpected output messages?

```
Installing ocramius/package-versions (1.1.3)

[...]

ocramius/package-versions:  Generating version class...
ocramius/package-versions: ...done generating version class
```

Commits
-------

89336f6 Update project to Symfony 4 beta4

Author: javiereguiluz

.travis.yml

@@ -32,7 +32,7 @@ script:
   # this checks that the Twig template files contain no syntax errors
   - ./bin/console lint:twig templates
   # this checks that the XLIFF translations contain no syntax errors
-  - ./bin/console lint:xliff translations
+  - ./bin/console lint:xliff config/translations
   # this checks that the application doesn't use dependencies with known security vulnerabilities
   - ./bin/console security:check --end-point=http://security.sensiolabs.org/check_lock
   # this checks that the composer.json and composer.lock files are valid