explained tests dependency how to set up environment

When I went to run `bash build.sh`, I was getting this error:

    Couldn't create database for {"adapter"=>"postgresql", "database"=>"spree/frontend_spree_development", "username"=>"postgres", "min_messages"=>"warning"}
    FATAL:  role "postgres" does not exist

This [SO QA](http://stackoverflow.com/questions/7863770/rails-and-postgresql-role-postgres-does-not-exist) did the trick and I ran:

    createuser -s -r postgres

My rails dummy applications were now set up. Updated [Readme](https://github.com/spree/spree/blob/master/README.md) to contain this knowledge.

There was also an issue where I did not have PhantomJS in my path. I also specify that the tests require PhantomJS and how they can install it.

Hope this helps makes spree friendlier for people.