Fix vulnerability CVE-2018-3760 (ifmeorg#907)

Fix vulnerability by upgrading the gem using `bundle update sprockets`.

Author: rhiroyuki

Gemfile.lock

@@ -445,7 +445,7 @@ GEM
       tilt (~> 2.0)
     spring (2.0.2)
       activesupport (>= 4.2)
-    sprockets (3.7.1)
+    sprockets (3.7.2)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     sprockets-es6 (0.9.2)