Bump rack-contrib from 2.0.1 to 2.1.0

[dependabot-preview]

Bumps rack-contrib from 2.0.1 to 2.1.0.

Release notes

Sourced from rack-contrib's releases.

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.

Commits

• d2538f7 Bump ruby versions in travis to all-latest
• 33e5e01 Removing has_rdoc deprecation warning (#153)
• 008d283 Update rack-contrib.gemspec
• 7ad2de1 Merge pull request #76 from libis/allow_post_body_content_custom_parsing
• 2308a54 Ruby 2.5 support
• 74afe70 Merge pull request #149 from mvasin/patch-1
• edbd551 Fix typo
• 7fd98fc Merge pull request #147 from znz/update-README
• fa9cdaf Remove lines of removed components
• edf679d Proper merging and fixed tests
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #85.