Don't load Rails environment when compiling assets

I don't know why but Heroku has started complaining while compiling the
assets. This article[1] instructs you to set
`config.assets.initialize_on_precompile` to false to get around the problem.
There's more info in the Rails guide[2]

[1] https://devcenter.heroku.com/articles/rails-asset-pipeline#troubleshooting
[2] http://guides.rubyonrails.org/asset_pipeline.html#precompiling-assets