Adding json dependancy
diff --git a/predictionio.gemspec b/predictionio.gemspec
index bfc9e9b..13b651d 100644
--- a/predictionio.gemspec
+++ b/predictionio.gemspec
@@ -18,5 +18,5 @@
   s.platform = Gem::Platform::RUBY
   s.required_ruby_version = '>= 1.9.3'
   s.files = Dir[File.join('lib', '**', '**')]
-  s.add_runtime_dependency 'json', ["~> 1.8.0"]
+  s.add_runtime_dependency 'json', '~> 1.8'
 end