Fix test broken on ruby 1.9.3 and jruby

mime-types transistively required by rails, which version >=3 will require ruby >= 2.0

Author: zetachang

Diff for: reactrb.gemspec

@@ -30,6 +30,7 @@ Gem::Specification.new do |s|
 
   # For Test Rails App
   s.add_development_dependency 'rails', '4.2.4'
+  s.add_development_dependency 'mime-types', '< 3'
   s.add_development_dependency 'react-rails', '~> 1.3.0'
   s.add_development_dependency 'opal-rails', '~> 0.8.1'
   if RUBY_PLATFORM == 'java'