Make sure assets are precompiled on Heroku

Heroku recommends that `config.assets.initialize_on_precompile = false`,
which means that this initializer will not be invoked by
`rake assets:precompile`, which forces developers deploying to Heroku
to duplicate this initializer in their apps' configs.
Merge pull request #1910 from exviva/always_configure_precompile

Make sure assets are precompiled on Heroku