Merge branch 'rails4'

Conflicts:
	README.md
	lib/paper_trail/has_paper_trail.rb
	lib/paper_trail/version.rb
	test/dummy/config/initializers/paper_trail.rb
	test/unit/model_test.rb
	test/unit/version_test.rb
Adjusting Travis CI config to cover ActiveRecord 3 and 4 and allow failures when appropriate, tweaked 3.0.gemfile so it can handle ruby18 and jruby