2.1.5-RC2

This is a pre-release of 2.1.5 to make sure the new nebula-plugin-free release process works.

If the release fails at the bintray upload due to configuration mistakes, we'll adjust it an release RC3 and on. If it succeeds, that's fine as well because the final 2.1.5 will come in a couple of days.

RC1 failed due to wrong config: repo and name were not RxJava/RxJava.