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.
> If nightly builds are the best option right now, let's go for it. Just wanted to ask.

I think so, at least until a release is out.. there are no official packages by the Neovim team yet, so the ones linked at "Packaged install" might or might not be up-to-date. I'll let you know when the nightlies are ready.