- Removed test-unit dependency

- 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

test/readme_test.rb

@@ -1,7 +1,7 @@
 # Tests to check if all the README examples work.
 require File.expand_path('../helper', __FILE__)
 
-class ReadmeTest < Test::Unit::TestCase
+class ReadmeTest < Minitest::Test
   example do
     mock_app { get('/') { 'Hello world!' } }
     get '/'