remove travis.yml

[prabirshrestha]

Travis has been notoriously slow and moving to github actions seems to be very fast.

Some OS we currently don't run certain version of vim.

• Linux doesn't run vim v8.0 this is because the download url doesn't seem to match the version of v8.1 and v8.2 and we already have it running in windows and it quite old so shouldn't be a problem.
• mac doesn't run any vim tests and only runs neovim because I don't want the CI to be compiling vim and linux coverage should most likely be enough due to its roots with posix.
• I love how neovim releases are done, hence neovim runs on all OS - mac/linux/window along with nightly builds. Wished vim did the same instead of multiple repos.

[prabirshrestha]

Here is an example of slow: github actions already finished running all the OS/vim/neovim matrix and travis hasn't even started.