Log Message:
Update ruby-rack-contrib to 1.6.0.

1.6.0 (2017/09/01)

* Rack::PostBodyContentTypeParser: if the middleware is told a POST body is
  JSON, but it doesn't parse as JSON, then... it's not really JSON, and the
  request is now rejected with a 400 response. Thanks to Yukihiko SAWANOBORI
  (@sawanoboly) for the fix.

1.5.0 (2017/07/19)

After an extended hiatus, rack-contrib maintenance is back on track. This
is a tidy-up release, merging things that have sat around for far too long.

* git-version-bump has now been moved to being a development dependency,
  thanks to Tobias Haagen Michaelsen.

* Rack::AcceptLocale can be restricted to a set of enforced locales, thanks to
  Paco Guzman.

* Rack::NotFound's path argument is now optional, thanks to Ed Morley.

* Rack::BounceFavicon now has a description and tests, thanks to Steven
  Wilkin.

* The automated Travis CI suite now tests all supported Ruby versions up to
  2.4, which necessitated a few small changes.