Point pipeline status to the master branch.

Was pointing to the rtoy-gitlab-ci branch, but we want this to be the
status of the master branch.

README.md

-[![pipeline status](https://gitlab.common-lisp.net/cmucl/cmucl/badges/rtoy-gitlab-ci/pipeline.svg)](https://gitlab.common-lisp.net/cmucl/cmucl/commits/rtoy-gitlab-ci)
+[![pipeline status](https://gitlab.common-lisp.net/cmucl/cmucl/badges/master/pipeline.svg)](https://gitlab.common-lisp.net/cmucl/cmucl/commits/master)
 
 CMUCL is a free, high performance implementation of the Common Lisp
 programming language which runs on most major Unix platforms. It