Enable newrelic for only production and staging

Gemfile

@@ -21,7 +21,6 @@ gem 'upmin-admin'
 gem 'dotenv-rails'
 gem 'react-rails'
 gem 'react-rails-hot-loader'
-gem 'newrelic_rpm'
 group :development do
   gem 'better_errors'
   gem 'rubocop', require: false
@@ -65,3 +64,6 @@ group :test, :staging do
   gem 'launchy'
   gem 'selenium-webdriver'
 end
+group :production, :staging do
+  gem 'newrelic_rpm'
+end