don't initialize on precompile

I just did a fresh clone of the repo and followed the directions verbatim, but the deploy failed on the first deploy with:

```basb
       Running: rake assets:precompile
       rake aborted!
       could not connect to server: Connection refused
```

Which was fixed by adding this configuration to application.rb:

```ruby
config.assets.initialize_on_precompile = false
```

Reference: https://devcenter.heroku.com/articles/rails-asset-pipeline