Avoid built-in old test-unit

In Ruby <= 2.1, `test-unit` was shipped with Ruby itself (unbundling
was done for 2.2 -- see <https://bugs.ruby-lang.org/issues/9711>).

The `test-unit` version shipped with 2.1 breaks some of the tests.
To fix this, I've added the minimum needed version explicitly to the
`gemspec`, as well as added a `gems.rb` for allowing the use of
`bundler` to run the tests and ensure the correct `test-unit` is used.

Author: ivoanjo

gems.rb

@@ -0,0 +1,3 @@
+source 'https://rubygems.org'
+
+gemspec

ruby2_keywords.gemspec

@@ -19,4 +19,6 @@ Gem::Specification.new do |s|
   s.files = [
     "lib/ruby2_keywords.rb",
   ]
+  s.add_development_dependency 'test-unit', '>= 2.2'
+  s.add_development_dependency 'rake'
 end