[WIP] Travis: Run Vader tests.

Idea from https://github.com/neovim/neovim/issues/1687#issuecomment-67238982 by @justinmk.
That's just to speed up the build until this is finished, I'll remove this commit (https://github.com/fwalch/neovim/commit/76aa1dfde42bdaa56925aa727d15d206dadd0eb6) afterwards.