try not to load uglifier in prod, as it should be pre-compiled

Gemfile

@@ -17,7 +17,11 @@ gem 'sass-rails',
             ENV['PRECOMPILE'].to_s == '1')
 
 # Use Uglifier as compressor for JavaScript assets
-gem 'uglifier', '>= 1.3.0'
+# but do not load it in production (aka. passenger, as
+# we should have all js pre-compiled by then)
+gem 'uglifier', '>= 1.3.0',
+  require: (ENV['RAILS_ENV'].to_s.downcase != 'production' ||
+            ENV['PRECOMPILE'].to_s == '1')
 
 # Use CoffeeScript for .coffee assets and views
 gem 'coffee-rails'