Tests: newer Rubies need a gem to get test/unit

Thanks for reporting: @strzibny in #48, @ggilder in #50,
@davidcornu in #58, and @SamMolokanov in #60.

References #57.

fakeweb.gemspec

@@ -63,6 +63,10 @@ Gem::Specification.new do |s|
     s.add_development_dependency "json",              ["~> 1.7"]
   end
 
+  if RUBY_VERSION >= "2.2.0"
+    # Test::Unit is no longer distributed with Ruby as of 2.2.0
+    s.add_development_dependency "test-unit", ["~> 3.2.4"]
+  end
 
   if RUBY_PLATFORM == "java"
     s.add_development_dependency "jruby-openssl", ["~> 0.8"]