Use ActiveSupport::TestCase as base test class

Fixes Minitest base class warnings.

Minitest versions 4 and 5 have different test base classes.
ActiveSupport::TestCase handles this difference for us.
Specify minitest version for rails 3.2 test gemfile
Remove minitest development dependency

minitest is included by rails via activesupport, except for 3.2. For
3.2, we include it in the 3.2 gemfile.