Avoid sprocket building assets concurrently

This is a temporary fix until sass/sassc-ruby#207 is
resolved

This should stop the segfault from happening upon deploy

config/initializers/sprockets.rb.rb

@@ -0,0 +1,5 @@
+Rails.application.configure do
+  config.assets.configure do |env|
+    env.export_concurrent = false
+  end
+end