Add Travis CI

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

https://trello.com/c/pRpqWbkK
WIP: mappings
Add mappings for public API

https://trello.com/c/LRlRb93n
Only create mappings if user has not created them

https://trello.com/c/L41OwR4B