Cache locally-built Homebrew bottles in OSX builds

[native-api]

Closes #139.

• This PR doesn't fix OSX FFmpeg can no longer be built without GPL libs #142 , only addresses the immediate problem of failing builds.

It saves brew dependencies as they are built, measures elapsed time, and once it's high enough, fails the build but arranges for Travis cache to be saved. Then reuses the bottles on the next run. It doesn't seem that any solution currently exists, so rolled my own. Now, it does!
It also correctly detects updates and rebuilds bottles for new versions as needed.

So, basically, you need to restart jobs that print a message like "Building dependencies took too long" in Travis UI until they finish. As things are now, it takes 2 restarts to get a job to finish for the first time.
I'm not sure if Travis UI allows to restart jobs in a PR's automatic check -- in this PR, you'll be able to test this!

• Travis provides build stages to avoid the need for restarts, but with 2 restarts, the mac part of the build matrix would grow 3x. Since fixing OSX FFmpeg can no longer be built without GPL libs #142 will reduce build times drastically, I'm reluctant to make such changes until then.
  
  Travis looks for cache in the order: PR -> branch -> default branch. So building on master should make any other branch be able to find a cache and hopefully finish on the first try.

---

In other news: Other changes:

• Cached the Homebrew installation (but not the cellar) as well so it's not updated all the way each time.
• Set up ccache cache. This brings recompile times down from 20 to 8(!) minutes. Dunno how much it will help in the normal course of action 'cuz the effect depends on how many source files are unchanged between builds.
  • The default ccache cache size limit in 1GB. Thus it may cause cache uploading timeouts in perspective. I dunno how much data Travis' 3 min caching timeout translates into, so can't adjust limits myself upfront.
• Added -j<...> to make args since all Travis builders are 2-core. This only affects the project itself (brew uses its own make args which already include -j<...>, see brew --env output).

[skvark]

Nice! I will check this during weekend, I'm too busy today/tomorrow.

[native-api]

Didn't manage to enable ccache on both Linux and OSX: in Linux, creating /usr/local/Homebrew fails with EACCES, terminating caching logic. The only way is to replicate cache: directories: $HOME/.ccache in every Linux job entry which I presume would inconvenience you too much.

[skvark]

I will check this and the other pull requests / issues asap during this week, didn't found the time last weekend.

[native-api]

which I presume would inconvenience you too much.

On second thought, if - BDIST_PARAMS="-- -DCMAKE_OSX_DEPLOYMENT_TARGET:STRING=10.7" in every OSX entry (that really belongs in setup.py instead since compilation fails without it) don't inconvenience you, these probably won't, either.

[skvark]

About that deployment target param: it was a hack due to a bug in scikit-build (#110 (comment)). I think it can be moved safely to setup.py now.

[skvark]

I'm a bit afraid of the maintenance burden this might introduce, but I think there's currently no better way to do this. And the build times are so much better with this approach.

[native-api]

This PR doesn't fix #142

Shucks, Github detected the "fix" word, too, and closed #142 as well when you merged this!
Fortunately, it doesn't matter in this case :)

[skvark]

Github is trying to be too smart 😄