[skip ci] Update CI badge in readme
[skip ci] Update CI badge in readme

This trivial MR updates the CI badge in the readme to point to the
new CI on gitlab, rather than the very out-of-date badge from
Travis.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc