Add new ey.yml option `precompile_assets_command`

Setting `precompile_assets_command` overrides the asset precompile rake
command. Currently the default is:

`rake assets:precompile RAILS_GROUPS=assets`

Bundler binstubs are in PATH so gem binaries will load through bundler.
Merge pull request #113 from engineyard/precompile_assets_command

Add new ey.yml option `precompile_assets_command`