Adds PHP_CodeSniffer version constraints

[frenck]

Proposed Changes

From TravisCI build ouput:

$ composer validate
./composer.json is valid, but with a few warnings
See https://getcomposer.org/doc/04-schema.md for details on the schema
require.squizlabs/php_codesniffer : unbound version constraints (*) should be avoided

This PR adds version constraints to avoid that warning. Also, this plugin is currently only compatible with 2.x and 3.x of PHP_CodeSniffer.

Related Issues

n/a