Set assets.export_concurrent to false

There seems to be issues with Sprockets version 4, errors like [1]
occur when precompiling assets.

A suggestion I've seen to avoid this is to add this configuration [2].

1: SassC::SyntaxError: Error: error in C function font-path: undefined
method `[]' for nil:NilClass
2: rails/sprockets#581 (comment)

config/initializers/assets.rb

@@ -3,6 +3,9 @@
 # Version of your assets, change this if you want to expire all your assets.
 Rails.application.config.assets.version = "1.0"
 
+# TODO: This seems to help work around asset precompilation failures
+Rails.application.config.assets.export_concurrent = false
+
 # Add additional assets to the asset load path
 # Rails.application.config.assets.paths << Emoji.images_path