Bump sprockets from 3.3.4 to 3.7.2

Bumps [sprockets](http://getsprockets.org/) from 3.3.4 to 3.7.2.

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -50,6 +50,7 @@ GEM
       xpath (~> 2.0)
     chronic (0.10.2)
     coderay (1.0.9)
+    concurrent-ruby (1.1.5)
     crass (1.0.4)
     cucumber (1.3.20)
       builder (>= 2.1.2)
@@ -125,7 +126,7 @@ GEM
     omniauth-openid (1.0.1)
       omniauth (~> 1.0)
       rack-openid (~> 1.3.1)
-    rack (1.6.4)
+    rack (1.6.11)
     rack-openid (1.3.1)
       rack (>= 1.1.0)
       ruby-openid (>= 2.1.8)
@@ -191,8 +192,9 @@ GEM
       sprockets (>= 2.8, < 4.0)
       sprockets-rails (>= 2.0, < 4.0)
       tilt (>= 1.1, < 3)
-    sprockets (3.3.4)
-      rack (~> 1.0)
+    sprockets (3.7.2)
+      concurrent-ruby (~> 1.0)
+      rack (> 1, < 3)
     sprockets-rails (2.3.3)
       actionpack (>= 3.0)
       activesupport (>= 3.0)