Disable asset debugging mode in development

This was outputting an `application.debug-[fingerprint].js` file, which
was causing a lot of confusion. [This
issue](rails/jsbundling-rails#73) had the
solution for why it was happening. I don't think there's much benefit to
outputting the `.debug` file if we're going to minify the JS (coming in
the next commit), so I've turned it off.

config/environments/development.rb

@@ -58,7 +58,7 @@
   # Debug mode disables concatenation and preprocessing of assets.
   # This option may cause significant delays in view rendering with a large
   # number of complex assets.
-  config.assets.debug = true
+  config.assets.debug = false
 
   # Suppress logger output for asset requests.
   config.assets.quiet = true