Bump sprockets from 2.12.3 to 3.7.2

Gemfile.lock

@@ -64,6 +64,7 @@ GEM
       execjs
     coffee-script-source (1.10.0)
     colorize (0.7.5)
+    concurrent-ruby (1.1.5)
     crass (1.0.1)
     curb (0.8.6)
     diff-lcs (1.2.5)
@@ -83,7 +84,6 @@ GEM
     ffi (1.9.6-x86-mingw32)
     globalid (0.3.0)
       activesupport (>= 4.1.0)
-    hike (1.2.3)
     http-cookie (1.0.2)
       domain_name (~> 0.5)
     i18n (0.7.0)
@@ -104,7 +104,6 @@ GEM
     mime-types (2.4.3)
     mini_portile (0.6.2)
     minitest (5.5.1)
-    multi_json (1.10.1)
     net-http-digest_auth (1.4)
     net-http-persistent (2.9.4)
     net-scp (1.2.1)
@@ -124,7 +123,7 @@ GEM
     pg (0.18.1-x86-mingw32)
     puma (2.11.0)
       rack (>= 1.1, < 2.0)
-    rack (1.6.0)
+    rack (1.6.11)
     rack-cors (0.3.1)
     rack-test (0.6.3)
       rack (>= 1.0)
@@ -205,11 +204,9 @@ GEM
       sprockets-rails (>= 2.0, < 4.0)
       tilt (>= 1.1, < 3)
     sax-machine (1.3.0)
-    sprockets (2.12.3)
-      hike (~> 1.2)
-      multi_json (~> 1.0)
-      rack (~> 1.0)
-      tilt (~> 1.1, != 1.3.0)
+    sprockets (3.7.2)
+      concurrent-ruby (~> 1.0)
+      rack (> 1, < 3)
     sprockets-rails (2.2.4)
       actionpack (>= 3.0)
       activesupport (>= 3.0)
@@ -220,7 +217,7 @@ GEM
       net-ssh (>= 2.8.0)
     thor (0.19.1)
     thread_safe (0.3.4)
-    tilt (1.4.1)
+    tilt (2.0.10)
     tzinfo (1.2.2)
       thread_safe (~> 0.1)
     unf (0.1.4)