Ensure spec/rails_app/db/ is present by creating schema.rb

Aaron reports that specs do not run without spec/rails_app/db.  The
directory was created by "rails plugin new" but Git didn't track it
because it was empty.

Add a file to db/ so it is created automatically during checkout.  We
could add a .gitkeep, but schema.rb belongs there anyway, so create an
initial one with no tables for now.