Travis CI: Also run examples/ in neovim mode

This sets up neovim and runs the vroom examples under neovim mode to ensure we don't completely break neovim mode.

Fixes #64.
@fwalch Thanks for updating the setup and verifying!
What I'd like to do is run against the latest packaged neovim, i.e. what users will get if they follow the "Packaged Install" instructions at https://github.com/neovim/neovim/wiki/Installing. I also have the ulterior motive of #66: ideally we have a simple and non-brittle config we can recommend for plugin authors to test their vim plugins in vim and neovim.

If nightly builds are the best option right now, let's go for it. Just wanted to ask.