[Security] Bump sprockets from 2.12.4 to 3.7.2

Bumps [sprockets](http://getsprockets.org/) from 2.12.4 to 3.7.2. **This update includes security fixes.**

Signed-off-by: dependabot[bot] <support@dependabot.com>

Author: dependabot-support

Gemfile

@@ -26,7 +26,7 @@ gem 'equivalent-xml', '~> 0.6.0'
 gem 'jquery-rails', '~> 4.3.1'
 gem 'rails_autolink', '~> 1.1.6'
 gem 'qa', '~> 1.1.0'
-gem 'sprockets', '~> 2.12.4'  # locked
+gem 'sprockets', '~> 3.7.2'  # locked
 gem 'rack-dev-mark', '~> 0.7.5'
 
 # Deploy with Capistrano

Gemfile.lock

@@ -125,6 +125,7 @@ GEM
       coffee-script-source
       execjs
     coffee-script-source (1.10.0)
+    concurrent-ruby (1.0.5)
     coveralls (0.8.21)
       json (>= 1.8, < 3)
       simplecov (~> 0.14.1)
@@ -156,7 +157,6 @@ GEM
     globalid (0.3.7)
       activesupport (>= 4.1.0)
     hashie (3.5.6)
-    hike (1.2.3)
     hooks (0.3.6)
       uber (~> 0.0.4)
     http-cookie (1.0.2)
@@ -218,7 +218,6 @@ GEM
     mini_mime (0.1.3)
     mini_portile2 (2.3.0)
     minitest (5.10.2)
-    multi_json (1.12.1)
     multi_xml (0.6.0)
     multipart-post (2.0.0)
     net-scp (1.2.1)
@@ -267,7 +266,7 @@ GEM
       nokogiri (~> 1.6)
       rails (>= 4.2.0, < 6.0)
       rdf
-    rack (1.6.8)
+    rack (1.6.10)
     rack-dev-mark (0.7.5)
       rack (>= 1.1)
     rack-test (0.6.3)
@@ -389,11 +388,9 @@ GEM
       nokogiri
       stomp
       xml-simple
-    sprockets (2.12.4)
-      hike (~> 1.2)
-      multi_json (~> 1.0)
-      rack (~> 1.0)
-      tilt (~> 1.1, != 1.3.0)
+    sprockets (3.7.2)
+      concurrent-ruby (~> 1.0)
+      rack (> 1, < 3)
     sprockets-rails (2.3.3)
       actionpack (>= 3.0)
       activesupport (>= 3.0)
@@ -406,7 +403,7 @@ GEM
       tins (~> 1.0)
     thor (0.19.4)
     thread_safe (0.3.6)
-    tilt (1.4.1)
+    tilt (2.0.8)
     tins (1.13.3)
     tzinfo (1.2.3)
       thread_safe (~> 0.1)
@@ -484,7 +481,7 @@ DEPENDENCIES
   sitemap_generator (~> 5.3.1)
   solrizer (~> 3.1.0)
   solrizer-fedora!
-  sprockets (~> 2.12.4)
+  sprockets (~> 3.7.2)
   uglifier (~> 3.2.0)
   unicorn (~> 5.3.0)