unicorn.gemspec: depend on test-unit 3.0
test-unit 3 and minitest 5 will have equal support status as a
bundled gems when Ruby 2.2.0 is released in December 2014. These
bundled gems will appear in the user-oriented tarball installations,
but do not get installed by "make install" when installing Ruby
from SVN or git.
test-unit appears to be actively maintained and good at keeping
backwards compatibility even on a major version change, so this
means no code changes on our end. I am not convinced switching to
minitest is worth the effort.
Cc: Ken Dreyer <ktdreyer@ktdreyer.com>