Pin test-unit gem to ensure CI works on Ruby 1.9

[mattbrictson]

The test-unit gem recently added Ruby 2 keyword arguments to its implementation. This breaks our CI tests when running on Ruby 1.9, with errors like this:

/home/runner/work/plist/plist/vendor/bundle/ruby/1.9.1/gems/test-unit-3.6.7/lib/test/unit.rb:3:in `require':
/home/runner/work/plist/plist/vendor/bundle/ruby/1.9.1/gems/test-unit-3.6.7/lib/test/unit/testcase.rb:580:
syntax error, unexpected tLABEL (SyntaxError)
      def run(result, runner_class: nil)
                                   ^

Fix by pinning to an older version of the test-unit gem when running on Ruby 1.9.