Envfile in newrelic_rpm-3.7.1.188

- old
+ new

@@ -6,10 +6,8 @@
 
 excon_versions.each do |excon_version|
   gemfile <<-RB
     gem 'excon', '~> #{excon_version}'
     gem 'rack'
-    if RUBY_VERSION < '1.9.0'
-      gem 'json'
-    end
+    gem 'json', :platforms => [:rbx, :mri_18]
   RB
 end