We use minitest for Sinatra's test suite but we weren't using its rake task. I've updated the Rakefile to require and use Minitest default rake task to simplify.
Another change is to rename the `helper.rb` file to `test_helper.rb` because I think that name is used more in the community and require it directly without calling `File.expand_path`
- updated minitest to 5.0
- Removed Test::Unit::TestCase and started using Minitest::Test instead
- Fixed usage of assert_raise
- Fixed usage of refute_nil
- Fixed and removed usage of assert_nothing_raised
