Extract logic to MiniTest::Rails::Testing

Use MiniTest::Rails::Testing.default_tasks instead of MINITEST_TASKS.
Override ActiveSupport::TestCase by default

This is a big change that simplifies the approach to how minitest is integrated.
Use ActiveSupport::TestCase instead of MiniTest::Rails::ActiveSupport::TestCase.
Instead of maintaining separate TestCase objects, overwrite the standard objects.
Focus only on on what is needed to enable minitest and the spec DSL.
Remove the MiniTest::Rails:: prefix from the generated tests.
Update gemfiles

Use the most recent versions with the recent security fixes.
Add early support for Rails 4