Test against custom vim versions in Travis CI

Since maktaba strives for broad vim support and backwards compatibility, we should be testing against the oldest vim version we intend to support and possibly the latest development version of vim (with `allow_failures` on the latest dev version if it proves to be too flaky).

If we can find pre-built packages for specific versions of vim, that would be best. Cloning and building vim in the Travis jobs would probably be slow and wasteful.