Bump dependencies

This updates all non-Active Record 4 dependencies. I went with separate commits so that any regressions can be git-bisected down to a particular update, but I’m happy to squash them if that’s preferred.

Still outdated gems (Active Record 4 and its non-upgradable deps):
  * activemodel (4.0.0 > 3.2.13)
  * activerecord (4.0.0 > 3.2.13)
  * activesupport (4.0.0 > 3.2.13)
  * arel (4.0.0 > 3.0.2)
  * builder (3.2.2 > 3.0.4)
  * i18n (0.6.4 > 0.6.1)
  * tzinfo (1.0.1 > 0.3.37)