Fix migrators not adding an extension in Rails 4.0.

Bump version to 1.4.2
Backport fix from master to minitest version in new Rubies.
Fix minitest version in Rails-specific Gemfiles.
Merge branch '1-4-stable'

Conflicts:
	CHANGELOG.md
	gemfiles/Gemfile.rails-3.X
	gemfiles/Gemfile.rails-4.0
	lib/generators/public_activity/migration_upgrade/migration_upgrade_generator.rb
	lib/public_activity/version.rb