Run vim8 Travis configuration

[dbarnett]

Vroom tests are running in a Travis environment with vim version 7 installed, so tests for vim8-specific functionality like #184 need to be skipped in Travis. We should see if we can include a vim8 configuration in the matrix and enable those tests for the vim8 configuration.

[dbarnett]

I found https://github.com/junegunn/vim-plug runs vim8 on Travis, but they do it by building vim from source on each Travis run which makes their Travis runs very slow (junegunn/vim-plug#176). If there were a prebuilt vim available, it would benefit us and vim-plug.

For comparison, see Neovim's solution at https://github.com/neovim/bot-ci/blob/master/scripts/travis-setup.sh and https://github.com/neovim/bot-ci#generated-builds.