Use --startas over --exec

The --exec option does checking for the process name when starting but
this interacts poorly with interpreted languages. In addition process
run status is controlled by using the --pidfile flag. The --startas flag
will run the specified binary but doesn't do any unneeded checking.
chdir into unicorn approot when starting app

This is building towards support for running unicorn via bundler. For
bundler to be used we have to be in the approot for the Gemfile.