fix builds on ruby 1.8.7 and 1.9.3

- drop rubocop from ruby 1.9.x builds as bundler doesn't allow the expresiveness needed for specifying different versions based on the platform. (see rubygems/bundler#751)

Gemfile

@@ -16,9 +16,15 @@ group :test do
   gem 'rack', '~> 1.2', :platforms => [:jruby_18, :jruby_19, :ruby_18, :ruby_19, :ruby_20, :ruby_21]
   gem 'rest-client', '~> 1.6.0', :platforms => [:jruby_18, :ruby_18]
   gem 'rspec', '>= 3'
-  gem 'rubocop', '>= 0.37', :platforms => [:ruby_19, :ruby_20, :ruby_21]
+  gem 'rubocop', '>= 0.37', :platforms => [:ruby_20, :ruby_21]
   gem 'simplecov', '>= 0.9'
   gem 'yardstick'
+
+  platforms :ruby_18, :ruby_19 do
+    gem 'json', '< 2.0'
+    gem 'tins', '< 1.7'
+    gem 'term-ansicolor', '< 1.4.0'
+  end
 end
 
 gemspec