Tidy up gemspec and update dependency versions.

Loosen the dependency constraints too.
Add test-unit to the deps so we can test in ruby 2.2.
Setup gem caching for Travis.
Test using ruby 2.1.5 in Travis.
Merge branch 'next'

* next:
  Test using ruby 2.1.5 in Travis.
  Setup gem caching for Travis.
  Add test-unit to the deps so we can test in ruby 2.2.
  Tidy up gemspec and update dependency versions.