Using .ruby-version file
Use Heroku API to run migrations

This addresses issue #13. Using the Heroku API directly instead of
making a system call to run the migrations solves my problem and seems
like the right way to handle migrations anyways.
Adding Rendezvous to support blocking ps ops

The Rendezvous gem from geemus was created to read the output of heroku
api commands which return immediately. This commit passes the attach:
true option to post_ps which makes the process return a rendezvous URL
that can be provided to Rendezvous to read from a socket to the URL and
print the output that it returns.  This has the effect of making the
rake db:migrations command block until the migrations have finished
running.
Moving Rendezvous into class via dep injection