[gemspec] Depend on pure Ruby version of json gem.

Gemfile.lock

@@ -7,12 +7,12 @@ GIT
 
 GIT
   remote: https://github.com/CocoaPods/Core.git
-  revision: 96db7cc599bae51ae37ad9c5800bca34b7a3f837
+  revision: 0cbab17f071fa791faab6a65b0369709f8c103bf
   branch: master
   specs:
     cocoapods-core (0.27.0)
       activesupport (>= 3.2.15, < 4)
-      json (~> 1.8)
+      json_pure (~> 1.8)
       nap (~> 0.5)
 
 GIT
@@ -57,7 +57,7 @@ PATH
       cocoapods-downloader (~> 0.2.0)
       colored (~> 1.2)
       escape (~> 0.0.4)
-      json (~> 1.8)
+      json_pure (~> 1.8)
       open4 (~> 1.3)
       xcodeproj (~> 0.14.0)
 
@@ -82,7 +82,7 @@ GEM
     ffi (1.9.0)
     github-markup (0.7.5)
     i18n (0.6.5)
-    json (1.8.1)
+    json_pure (1.8.1)
     listen (1.1.6)
       rb-fsevent (>= 0.9.3)
       rb-inotify (>= 0.9)

cocoapods.gemspec

@@ -34,7 +34,7 @@ s.files = Dir["lib/**/*.rb"] + %w{ bin/pod bin/sandbox-pod README.md LICENSE CHA
 
   s.add_runtime_dependency 'colored',       '~> 1.2'
   s.add_runtime_dependency 'escape',        '~> 0.0.4'
-  s.add_runtime_dependency 'json',          '~> 1.8'
+  s.add_runtime_dependency 'json_pure',     '~> 1.8'
   s.add_runtime_dependency 'open4',         '~> 1.3'
   s.add_runtime_dependency 'activesupport', '>= 3.2.15', '< 4'