[readme] Update build badge

dumux-lecture is currently tested alongside dumux. So they share the same build badge.

README.md

@@ -39,7 +39,7 @@ See the file `LICENSE.md` for full copying permissions.
 Automated Testing
 -----------------
 
-[![build status](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux-lecture/badges/master/build.svg)](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux-lecture/commits/master)
+[![buildbot badge](https://git.iws.uni-stuttgart.de/buildbot/badges/dumux-master-dune-rel-2-7-clang-full.svg)](https://git.iws.uni-stuttgart.de/buildbot/#/builders)
 
 The simulations in DuMuX-Lecture are tested automated using buildbot.
 The test suite consists of the actual simulations adjusted to smaller run times.