Reduced sprockets to version 3.x

Signed-off-by: Jim Martens <github@2martens.de>

Gemfile

@@ -26,7 +26,7 @@ group :jekyll_plugins do
   gem 'jekyll-sitemap'
   gem 'jekyll-titles-from-headings'
   gem 'jemoji'
-  gem 'sprockets', '~> 4.0.beta'
+  gem 'sprockets'
   gem 'uglifier'
 end
 gem 'jekyll-sass-converter'

Gemfile.lock

@@ -203,7 +203,7 @@ GEM
     sawyer (0.8.1)
       addressable (>= 2.3.5, < 2.6)
       faraday (~> 0.8, < 1.0)
-    sprockets (4.0.0.beta6)
+    sprockets (3.7.1)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     terminal-table (1.8.0)
@@ -266,7 +266,7 @@ DEPENDENCIES
   mercenary
   minima
   rouge
-  sprockets (~> 4.0.beta)
+  sprockets
   terminal-table
   uglifier