Merge branch 'upgrade-sprockets' into 'master'

Update sprockets to 3.6.0

Sprockets 3.5.0 reintroduces GZIP-ing and 3.6.0 includes performance improvements.
Changelog: https://github.com/rails/sprockets/blob/3.x/CHANGELOG.md

Closes #14344.

See merge request !3544

Gemfile

@@ -8,7 +8,7 @@ gem 'responders', '~> 2.0'
 
 # Specify a sprockets version due to increased performance
 # See https://gitlab.com/gitlab-org/gitlab-ce/issues/6069
-gem 'sprockets', '~> 3.3.5'
+gem 'sprockets', '~> 3.6.0'
 
 # Default values for AR models
 gem "default_value_for", "~> 3.0.0"

Gemfile.lock

@@ -776,7 +776,8 @@ GEM
       spring (>= 0.9.1)
     spring-commands-teaspoon (0.0.2)
       spring (>= 0.9.1)
-    sprockets (3.3.5)
+    sprockets (3.6.0)
+      concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     sprockets-rails (3.0.4)
       actionpack (>= 4.0)
@@ -1033,7 +1034,7 @@ DEPENDENCIES
   spring-commands-rspec (~> 1.0.4)
   spring-commands-spinach (~> 1.0.0)
   spring-commands-teaspoon (~> 0.0.2)
-  sprockets (~> 3.3.5)
+  sprockets (~> 3.6.0)
   state_machines-activerecord (~> 0.3.0)
   task_list (~> 1.0.2)
   teaspoon (~> 1.1.0)