Restore Travis builds

[mourner]

@mikolalysenko I see that the travis build was disabled at some point, although the config is still in the repo — do you remember why? Can we reenable it?

[mikolalysenko]

I'm not sure what happened and it has been long enough that I can't remember why it is disabled. It should probably be turned back on.

[mourner]

@mikolalysenko looks like Travis is enabled and running fine (https://travis-ci.org/stackgl/headless-gl/builds), it's just that the GitHub hook is not there for some reason. Should a minute to add back (I don't have the necessary repo rights).