Bump sprockets from 3.5.2 to 3.7.2

Bumps [sprockets](https://github.com/rails/sprockets) from 3.5.2 to 3.7.2.
- [Release notes](https://github.com/rails/sprockets/releases)
- [Changelog](https://github.com/rails/sprockets/blob/v3.7.2/CHANGELOG.md)
- [Commits](rails/sprockets@v3.5.2...v3.7.2)

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

Author: dependabot[bot]

Gemfile.lock

@@ -60,7 +60,7 @@ GEM
       coffee-script-source
       execjs
     coffee-script-source (1.10.0)
-    concurrent-ruby (1.0.0)
+    concurrent-ruby (1.1.5)
     countries (1.2.5)
       currencies (~> 0.4.2)
       i18n_data (~> 0.7.0)
@@ -108,7 +108,7 @@ GEM
       activemodel (>= 4.0.1, < 5.0)
     quiet_assets (1.1.0)
       railties (>= 3.1, < 5.0)
-    rack (1.6.4)
+    rack (1.6.11)
     rack-test (0.6.3)
       rack (>= 1.0)
     rails (4.2.5.1)
@@ -176,7 +176,7 @@ GEM
       tilt (>= 1.3.3, < 2.1)
     sort_alphabetical (1.0.2)
       unicode_utils (>= 1.2.2)
-    sprockets (3.5.2)
+    sprockets (3.7.2)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     sprockets-rails (3.0.1)