Spring has sprung so let's make a new release

New features:

• Rack::PostBodyContentTypeParser -- you can now pass a block to the
  middleware to override the default "parse me some JSON" behaviour. Thanks
  to Kris Dekeyser (@Kris-LEBIS) for the patch.

• Ruby 2.5 support -- we're now running the test suite through Ruby
  2.5.1, as well as the latest patch releases of all other Ruby releases
  supported by rack-contrib (back to 2.2, the same as Rack itself). The
  only "interesting" change here is that some Rack::Profiler printers no
  longer work, which is not our fault, but rather a problem with
  ruby-prof.

Bug fixes:

• Remove a deprecation warning about has_rdoc. Thanks to Luciano Sousa
  (@lucianosousa) for the patch.