Updated jekyll assets to 3.0.12 and sprockets to 4.0

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

Gemfile.lock

@@ -274,7 +274,7 @@ GEM
     sawyer (0.8.1)
       addressable (>= 2.3.5, < 2.6)
       faraday (~> 0.8, < 1.0)
-    sprockets (3.7.2)
+    sprockets (4.0.0.beta8)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     sprockets-rails (3.2.1)
@@ -304,7 +304,7 @@ DEPENDENCIES
   bootstrap (~> 4.3.1)
   html-proofer
   jekyll (~> 3.8.4)
-  jekyll-assets (~> 3.0.9)
+  jekyll-assets (~> 3.0.12)
   jekyll-avatar
   jekyll-coffeescript
   jekyll-commonmark-ghpages
@@ -347,7 +347,7 @@ DEPENDENCIES
   rouge
   rubocop
   rubyzip (~> 1.2.2)
-  sprockets
+  sprockets (~> 4.0.0beta8)
   terminal-table
   uglifier