Use precompiled assets if present

This change uses precompiled assets if they are available.

The prior behavior was to ask the asset pipeline to compile the asset each time, which is what `Rails.application.assets.find_asset` unfortunately does.

Adding this feature allows premailer-rails3 to work correctly when assets are precompiled and all asset-compilation gems are included only in the `:assets` group in the bundle. 

Without this change, I have to include sass in my production slug on Heroku or I get an error: `uninitialized constant Sass::VERSION`.