Remove warning under Minitest 5

Under Minitest 5 (or a Rails 4.1 project), you will get a warning

    MiniTest::Unit::TestCase is now Minitest::Test.

when you attempt to run tests. This is because shoulda-matchers requires
'test/unit/testcase', even if Test::Unit is not being used.

It is not a good idea for shoulda-matchers to require *anything* that
the user may not be using, whether it's Test::Unit or RSpec. We should
assume that by the time shoulda-matchers is required, the user has
already required all of whatever test framework they are using, not none
or even some of that framework.
Add support to rails 4.1.0
Fix typo
Add missing description to #permit matcher