Updates to Minitest 5.0 and more

This updates tc-test to Minitest 5.0. Unfortunately, we had to remove `minitest-reporters` but we've opened https://github.com/newleaders/thincloud-test/issues/27 to track when it is again Minitest compatible.

* CI settings are updated
* Now uses test_helper.rb instead of minitest_helper.rb (remember to update your projects!)
* Gems updated all around
* Add some rake task helpers
* Add cane rake task
* Default rake task now runs cane
* Clean up of generator
* Removed Simplecov configuration from the generated test_helper.rb. Now we use a require `thincloud/test/simplecov` in `thincloud/test` where the simplecov.rb file contains the default configuration. README has been updated with details on how to use this.