Bump sprockets from 2.12.3 to 2.12.5

Bumps [sprockets](https://github.com/rails/sprockets) from 2.12.3 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@v2.12.3...v2.12.5)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile.lock

@@ -90,7 +90,7 @@ GEM
       sprockets-helpers (~> 1.1.0)
       sprockets-sass (~> 1.3.0)
     minitest (5.5.1)
-    multi_json (1.11.0)
+    multi_json (1.14.1)
     net-sftp (2.1.2)
       net-ssh (>= 2.6.5)
     net-ssh (2.9.2)
@@ -101,7 +101,7 @@ GEM
     padrino-support (0.12.5)
       activesupport (>= 3.1)
     ptools (1.3.2)
-    rack (1.6.0)
+    rack (1.6.13)
     rack-livereload (0.3.15)
       rack
     rack-test (0.6.3)
@@ -113,7 +113,7 @@ GEM
     sass (3.4.25)
     sassc (2.3.0)
       ffi (~> 1.9)
-    sprockets (2.12.3)
+    sprockets (2.12.5)
       hike (~> 1.2)
       multi_json (~> 1.0)
       rack (~> 1.0)