fix rack-proxy dependency in production

Gemfile

@@ -219,6 +219,8 @@ gem 'chronic', '~> 0.10.2'
 gem 'chronic_duration', '~> 0.10.6'
 
 gem 'webpack-rails', '~> 0.9.9'
+gem 'rack-proxy', '~> 0.6.0'
+
 gem 'sass-rails', '~> 5.0.6'
 gem 'coffee-rails', '~> 4.1.0'
 gem 'uglifier', '~> 2.7.2'
@@ -311,8 +313,6 @@ group :development, :test do
   gem 'activerecord_sane_schema_dumper', '0.2'
 
   gem 'stackprof', '~> 0.2.10'
-
-  gem 'rack-proxy', '~> 0.6.0'
 end
 
 group :test do