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.
That would require this PR do do something like https://github.com/fwalch/vroom/compare/google:travis_neovim...travis_neovim-nightly, which [seems to work](https://travis-ci.org/fwalch/vroom/builds/38186176). The nightly builds are not yet finished, though (see neovim/bot-ci#25).

The problem is that the runtime won't be found by default, that's why I set the `VIM` environment variable explicitly. I tried to create a [wrapper script](https://github.com/fwalch/bot-ci/blob/nightly/templates/nightly-wrapper.sh) for that, but that's not a clean solution either.