v5.1.0

Rainbows! 5.1.0 - rack 2.x compat, EM fixes

This release now supports rack 2.x along with existing rack 1.x
applications.  Additionally, we now wait for deferred actions
for EventMachine users on graceful shutdown.

Thanks to Julia López Aladro and <alex0375@gmail.com> for
bringing up these changes in the open-to-all
rainbows-public@bogomips.org mailing list:

  https://bogomips.org/rainbows-public/CAPhSs6OAU+motd+MF=WOSek9==StLjEQJsNwAO3-aRWCZzhs=g@mail.gmail.com/T/

  https://bogomips.org/rainbows-public/CAKwvcL-VH3we4qA1pkNAstTmWvqNA=Rir2N_YiWztV_qbaLQvA@mail.gmail.com/T/

There's also a few documentation and test fixes.

9 changes since Rainbows! 5.0.0:

      doc: bump olddoc to ~> 1.2 for extra NNTP URLs
      doc: move homepage and update URLs to HTTPS
      doc: Static_Files: remove Future section
      tests: use correct file:// URL for random_blob path
      support rack 2.x
      Revert "t/t0044-autopush.sh: remove test"
      tests: re-enable EventMachine tests, again
      eventmachine: wait for deferred actions to finish
      doc: additional disclaimers