Bump release to 2.1-M4
Bump release to 2.1-M4

blankazucenalg/lenskit,aglne/lenskit,vaibhav345/lenskit,kluver/lenskit,binweiwu/lenskit,martinlaz/lenskit,kluver/lenskit,kluver/lenskit,tajinder-txstate/lenskit,vijayvani/Lenskit,blankazucenalg/lenskit,martinlaz/lenskit,amaliujia/lenskit,linjunleo/lenskit,tajinder-txstate/lenskit,vaibhav345/lenskit,vaibhav345/lenskit,kluver/lenskit,tajinder-txstate/lenskit,vijayvani/Lenskit,binweiwu/lenskit,chrysalag/lenskit,aglne/lenskit,linjunleo/lenskit,amaliujia/lenskit,chrysalag/lenskit,tajinder-txstate/lenskit,kluver/lenskit,vaibhav345/lenskit