Bump sprockets from 2.10.1 to 2.12.5

Bumps [sprockets](https://github.com/rails/sprockets) from 2.10.1 to 2.12.5.
- [Release notes](https://github.com/rails/sprockets/releases)
- [Changelog](https://github.com/rails/sprockets/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rails/sprockets/compare/v2.10.1...v2.12.5)

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

Gemfile.lock

@@ -65,7 +65,7 @@ GEM
       treetop (~> 1.4.8)
     mime-types (1.25.1)
     minitest (4.7.5)
-    multi_json (1.8.2)
+    multi_json (1.14.0)
     multi_xml (0.5.5)
     multipart-post (1.2.0)
     nokogiri (1.5.10)
@@ -77,7 +77,7 @@ GEM
       multi_xml (~> 0.5)
       rack (~> 1.2)
     polyglot (0.3.3)
-    rack (1.5.2)
+    rack (1.5.5)
     rack-test (0.6.2)
       rack (>= 1.0)
     rails (4.0.1)
@@ -96,7 +96,7 @@ GEM
     rake (10.1.0)
     rdoc (4.0.1)
       json (~> 1.4)
-    sprockets (2.10.1)
+    sprockets (2.12.5)
       hike (~> 1.2)
       multi_json (~> 1.0)
       rack (~> 1.0)