Travis: Fail if files change after gulp

[RunDevelopment]

A lot of times, contributors (myself included) forgot to build or commit the minified files.

Since Travis is running gulp anyway, why not check for changes?
The current gulp build process is deterministic and only depends on the input files, so successive build won't change any files. So if files do change, we know that someone forgot to rebuild Prism.

We should be able to use git status to detect the changed files.