Enable neovim mode tests in Travis CI

Adds a Travis CI job configuration to run vroom in neovim mode on Glaive.

This will also serve as the basis for the recommended Travis CI configuration for neovim mode to be added to the vroom documentation (google/vroom#66). It demonstrates a few areas for improvement like the hard-coded vroom version number and awkward syntax for multiple Travis CI job configurations until Travis CI supports multiple jobs natively.