Address RubyGem build warnings

Author: jdennes

createsend.gemspec

@@ -4,15 +4,15 @@ require 'bundler/version'
 require File.expand_path('lib/createsend/version')
 
 Gem::Specification.new do |s|
-  s.add_runtime_dependency 'json', '>= 0'
+  s.add_runtime_dependency 'json', '~> 1.0'
   s.add_runtime_dependency 'hashie', '~> 3.0'
   s.add_runtime_dependency 'httparty', '~> 0.10'
   s.add_development_dependency 'rake', '~> 10.0'
   s.add_development_dependency 'fakeweb', '~> 1.3'
   s.add_development_dependency 'jnunemaker-matchy', '~> 0.4'
   s.add_development_dependency 'shoulda-context', '~> 1.2'
-  s.add_development_dependency 'simplecov'
-  s.add_development_dependency 'coveralls'
+  s.add_development_dependency 'simplecov', '~> 0'
+  s.add_development_dependency 'coveralls', '~> 0'
   s.name = "createsend"
   s.author = "James Dennes"
   s.description = %q{Implements the complete functionality of the Campaign Monitor API.}