Allow head Ruby versions to fail in Travis.

- Silencing a warning
- Fixing a test.
Tracking object usage.

- Found in github/mime-types at a504206b.
Reorganize benchmarking.
Return an enumerator when no block is given.

- I had apparently missed this lesson of being a good Ruby citizen over
  the last twelve years or so. Regardless, it's a damned fine idea.

- Found in SamSaffron/mime-types at bdc7c442.
Improving the performance of MIME::Types#type_for

- The performance improvement is about 20–25%.
Unsort extensions, restore old sort order.

Resolves #71.
Adding MIME::Type#friendly.
Adding reg, ps1, and vbs extensions.

Fix #69.
Documenting convert methods in support.