Change to restrict json version on Ruby 2.0

Gemfile

@@ -1,16 +1,16 @@
 source "https://rubygems.org"
 
-git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
-
 gemspec
 
+gem "json", "2.4.1" if RUBY_VERSION == "2.0.0"
+
 group :test do
   gem "activesupport"
   gem "benchmark-ips", "~> 2.7.2"
   gem "simplecov", "~> 0.16.1"
   gem "coveralls", "~> 0.8.22"
-  if RUBY_VERSION.split(".")[1].to_i > 0
-    gem "rspec-benchmark"
+  if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new("2.1.0")
+    gem "rspec-benchmark", "~> 0.6"
   end
 end