Move dependency install to build step not before first run

[icarito]

What happened just before the problem occurred

I attempted to deploy container in Google Cloud and it became stuck trying to compile nokogiri on the final container. This really should be done during build step. It was moved to a new start script in order to speed up Travis runs, which is a lofty goal and so was accepted, but it should not go in detriment of deployability, containers should start as quick as possible even if the image is larger, dependencies should come with them.

[icarito]

@alaxalves @jywarren a heads up that I'll try to fix this as it is blocking our migration.