Update README with some more Ubuntu detail

I'm not following the Vagrant easy way out. This updates the
instructions for a fresh install on Ubuntu 14.04.

Opinionatedly, I put rbenv instructions in here. It seems to make sense
to suggest that for developers, so the project owners can better manage
the ruby environment amongst the developer community.
Add PGUSER variable to initial schema load

Otherwise it uses the system user, which is probably not "opencongress".
Also, this raises an error if the command fails so that the migration is
run again if retrying the `bundle exec rake db:migrate`