Add Travis CI

Before running specs, we install `vim-gnome` to get a version compiled with
`+clientserver`.

https://trello.com/c/pRpqWbkK
Use `rspec` because `rake` doesn't take arguments
Vastly improve the README

https://trello.com/c/GeYamqK0
Promote vim-tmux-runner
Correctly explain VtrSendCommand