allow ruby-json 2.0.1 in the archive

--- a/activesupport/activesupport.gemspec
+++ b/activesupport/activesupport.gemspec
@@ -21,7 +21,7 @@
   s.rdoc_options.concat ['--encoding',  'UTF-8']
 
   s.add_dependency 'i18n',       '~> 0.7'
-  s.add_dependency 'json',       '~> 1.7', '>= 1.7.7'
+  s.add_dependency 'json',       '>= 1.7.7'
   s.add_dependency 'tzinfo',     '~> 1.1'
   s.add_dependency 'minitest',   '~> 5.1'
   s.add_dependency 'thread_safe','~> 0.3', '>= 0.3.4'