Log Message:
Update ruby-mime-types to 3.1.

== 3.1 / 2016-05-22

* 1 documentation change:

  * Tim Smith (@tas50) updated the build badges to be SVGs to improve
    readability on high-density (retina) screens with pull request
    {#112}[https://github.com/mime-types/ruby-mime-types/pull/112].

* 3 bug fixes

  * A test for MIME::Types::Cache fails under Ruby 2.3 because of frozen
    strings, {#118}[https://github.com/mime-types/ruby-mime-types/issues/118].
    This has been fixed.

  * The JSON data has been incorrectly encoded since the release of mime-types
    3 on the +xrefs+ field, because of the switch to using a Set to store
    cross-reference information. This has been fixed.

  * A tentative fix for
    {#117}[https://github.com/mime-types/ruby-mime-types/issues/117] has been
    applied, removing the only circular require dependencies that exist (and
    for which there was code to prevent, but the current fix is simpler).
    I have no way to verify this fix and depending on how things are loaded by
    +delayed_job+, this fix may not be sufficient.

* 1 governance change

  * Updated to {Contributor Covenant 1.4}[Code-of-Conduct_rdoc.html].