Update required ruby version to 2.0

Frenetic now uses keyword arguments which are only available in Ruby 2.

Author: Derek Lindahl

frenetic.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |gem|
   gem.name          = "frenetic"
   gem.require_paths = ["lib"]
   gem.version       = Frenetic::VERSION
-  gem.required_ruby_version = '>= 1.9.3'
+  gem.required_ruby_version = '>= 2.0.0'
 
   gem.add_dependency             'faraday',             '>= 0.8'
   gem.add_dependency             'faraday_middleware',  '>= 0.8'