[RFC] Add better way to install travis dependencies

All dependencies for building/testing are built into an external github
repository. This script will install everything into /opt/neovim-deps, and by
having github as single point of failure, travis builds are now very reliable.

From my POV this is a good thing, because we can rest assured that travis builds won't fail due to broken external servers. It will also speed up travis builds.

@jszakmeister what are your thoughts? Since you are handling build-related issues I leave this to your decision. If you like this solution, feel free to bundle it with #348