Fix Encoding::UndefinedConversionError

This prevents certain kind of encoding issues from being thrown by Ruby. This probably started with the change in https://github.com/colszowka/simplecov/commit/6abb79e.

### Changes
* Updates to Gemfile/Rakefile so various ruby versions all work
* Bumped Simplecov version to 0.8.2 to elicit bad behavior
* Updated fixture and added test for issue
* Added test as default rake task
* Updated conventions for current Minitest version