[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new CI on gitlab, rather than the very out-of-date badge from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.

README.md

 The Glasgow Haskell Compiler
 ============================
 
-[![Build Status](https://api.travis-ci.org/ghc/ghc.svg?branch=master)](http://travis-ci.org/ghc/ghc)
+[![pipeline status](https://gitlab.haskell.org/ghc/ghc/badges/master/pipeline.svg?style=flat)](https://gitlab.haskell.org/ghc/ghc/commits/master)
 
 This is the source tree for [GHC][1], a compiler and interactive
 environment for the Haskell functional programming language.