Upgrade middleman-sprockets gem

middleman now natifvely supports scss, so we only
need middleman-sprockets for coffeescript.
As of this upgrade, relying on middleman-sprockets for
CSS results in partials not being loaded--see
middleman/middleman-sprockets#127

So in this commit I also enable middleman-sprockets only for JS
so that CSS is processed directly by middleman.

Gemfile

@@ -16,7 +16,7 @@ gem 'mime-types'
 # Middleman Gems
 gem 'middleman', '>= 4.0.0'
 gem 'middleman-livereload'
-gem 'middleman-sprockets', '~> 4.0.0.rc.3'
+gem 'middleman-sprockets', '~> 4.1.1'
 gem 'middleman-s3_sync'
 
 gem 'sprockets', '~> 4.0.2'

Gemfile.lock

@@ -106,7 +106,7 @@ GEM
       parallel
       ruby-progressbar
       unf
-    middleman-sprockets (4.0.0)
+    middleman-sprockets (4.1.1)
       middleman-core (~> 4.0)
       sprockets (>= 3.0)
     mime-types (3.3.1)
@@ -159,7 +159,7 @@ DEPENDENCIES
   middleman (>= 4.0.0)
   middleman-livereload
   middleman-s3_sync
-  middleman-sprockets (~> 4.0.0.rc.3)
+  middleman-sprockets (~> 4.1.1)
   mime-types
   sprockets (~> 4.0.2)
   tzinfo-data

config.rb

@@ -64,7 +64,9 @@
   s3_sync.version_bucket             = false
 end
 
-activate :sprockets
+activate :sprockets do |sprockets|
+  sprockets.supported_output_extensions = ['.js']
+end
 
 caching_policy 'text/html', max_age: 0, must_revalidate: true
 default_caching_policy max_age: 60*60*24*365