Fix broken build.

As documented on https://github.com/shageman/the_next_big_thing/issues/18
the build is failing locally on my workstation, this fixes it.
Fix ruby version mismatch between main app and engines

As documented in: https://github.com/shageman/the_next_big_thing/issues/17
this is a temporary fix, we should discuss if we don't want to upgrade
to latest ruby.
Fix rvm not using the components gemset

As documented on:
https://github.com/shageman/the_next_big_thing/issues/16
this fix change the build script to force rvm to use (and create if necessary)
the component ruby-version and ruby-gemset.

Conflicts:
	build.sh