Upgrade sprockets

Gemfile

@@ -78,4 +78,3 @@ group :staging, :production do
   gem "rails_stdout_logging"
   gem 'rails_12factor'
 end
-

Gemfile.lock

@@ -110,7 +110,7 @@ GEM
     coderay (1.1.2)
     coercible (1.0.0)
       descendants_tracker (~> 0.0.1)
-    concurrent-ruby (1.0.5)
+    concurrent-ruby (1.1.5)
     connection_pool (2.2.1)
     crack (0.4.3)
       safe_yaml (~> 1.0.0)
@@ -228,7 +228,7 @@ GEM
       pry (>= 0.10.4)
     public_suffix (3.0.1)
     puma (3.11.2)
-    rack (2.0.4)
+    rack (2.0.7)
     rack-cors (1.0.2)
     rack-mini-profiler (0.10.7)
       rack (>= 1.2.0)
@@ -349,7 +349,7 @@ GEM
     simplecov-html (0.10.2)
     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)
@@ -470,4 +470,4 @@ RUBY VERSION
    ruby 2.4.0p0
 
 BUNDLED WITH
-   1.15.4
+   1.16.4