Update sprockets gem (#7858)

https://blog.heroku.com/rails-asset-pipeline-vulnerability

Gemfile.lock

@@ -567,7 +567,7 @@ GEM
       json (>= 1.8, < 3)
       simplecov-html (~> 0.10.0)
     simplecov-html (0.10.2)
-    sprockets (3.7.1)
+    sprockets (3.7.2)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     sprockets-rails (3.2.1)
@@ -764,4 +764,4 @@ RUBY VERSION
    ruby 2.5.0p0
 
 BUNDLED WITH
-   1.16.1
+   1.16.2