Require Ruby 2.0.0 or higher

Closes #944

Gemfile

@@ -1,5 +1,8 @@
 source 'https://rubygems.org'
 
+# Ruby 2.0 is the minimum requirement
+ruby ['2.0.0', RUBY_VERSION].max
+
 # Optional libraries.  To conserve RAM, comment out any that you don't need,
 # then run `bundle` and commit the updated Gemfile and Gemfile.lock.
 gem 'twilio-ruby', '~> 3.11.5'    # TwilioAgent