Insert before ActionDispatch::Static if available

Currently font_assets doesn't work with precompiled assets on Heroku because it's inserted after `ActionDispatch::Static`, which serves `/public` and precompiled assets in the absence of a standard web server. This pull changes the insert point to `ActionDispatch::Static` if it exists. This hits a very, very common Heroku use case and would solve i.e. rubymaverick/font_assets#10 and rubymaverick/font_assets#13.

`ActionDispatch::Static` is directly above `Rack::Cache` in the middleware stack, so this otherwise changes behavior very little.