Silence Faraday warning and Json missing gem on Ruby 1.8.7

1. Ruby version conditionals seem not to work in the .gemspec
2. spec.add_dependency "json" was left without the conditional
   (but in later Ruby versions it comes built-in) can someone test this in their machine?
3. Silences Faraday warning on Ruby 1.8
   When you run exercism on Ruby 1.8.7 faraday always gives the warning 
   >  Faraday: you may want to install system_timer for reliable timeouts