Bump sprockets from 2.12.4 to 2.12.5

Bumps [sprockets](https://github.com/rails/sprockets) from 2.12.4 to 2.12.5.
- [Release notes](https://github.com/rails/sprockets/releases)
- [Changelog](https://github.com/rails/sprockets/blob/master/CHANGELOG.md)
- [Commits](rails/sprockets@v2.12.4...v2.12.5)

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

Author: dependabot[bot]

Gemfile.lock

@@ -86,14 +86,14 @@ GEM
       middleman-core (>= 3.2)
       rouge (~> 1.0)
     minitest (5.9.0)
-    multi_json (1.12.1)
+    multi_json (1.15.0)
     padrino-helpers (0.12.8)
       i18n (~> 0.6, >= 0.6.7)
       padrino-support (= 0.12.8)
       tilt (~> 1.4.1)
     padrino-support (0.12.8)
       activesupport (>= 3.1)
-    rack (1.6.4)
+    rack (1.6.13)
     rack-livereload (0.3.16)
       rack
     rack-test (0.6.3)
@@ -107,7 +107,7 @@ GEM
     rouge (1.7.2)
     ruby18_source_location (0.2)
     sass (3.4.22)
-    sprockets (2.12.4)
+    sprockets (2.12.5)
       hike (~> 1.2)
       multi_json (~> 1.0)
       rack (~> 1.0)