Rainbows! 5.1.1

This release fixes an incompatibility introduced with
the release of unicorn 5.3.0, yesterday :x.

Thanks to Claudio Poli for reporting the bug (and reminding
me this project exists).

Two changes since 5.1.0 (2017-01-12):
      workaround for unicorn 5.3.0
      tests: update isolate deps for unicorn and raindrops