Change to restrict json version on Ruby 2.0

Author: piotrmurach

Diff for: Gemfile

@@ -5,6 +5,7 @@ gemspec
 if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new("2.1.0")
   gem "rspec-benchmark", "~> 0.6"
 end
+gem "json", "2.4.1" if RUBY_VERSION == "2.0.0"
 
 group :test do
   gem "benchmark-ips", "~> 2.8.4"