Set up CI tests

[fingolfin]

There should be CI (continuous integration) tests for this Homebrew tap / formulae.

Such a test should probably include brew lint, and it should try to install GAP via this formulae and run some tests with the resulting GAP (or perhaps brew can do that via brew test or something like that if it exists).

I googled a bit and this might help: https://github.com/danielbayley/homebrew-pipeline