upgrade the heroku-api wrapper (heroku.rb)
support PX dynos

This adds support for non-numeric dyno sizes. Currently only P is supported
size is a string
make ps:scale accept lowercase Xs too, as ps:resize does
update usage docs to mention PX dynos
ignore .rbenv-version
bump heroku-api to the released version
Merge branch 'master' into px-dynos
allow size to be an arbitrary word (string)

Leave the validation to the server side
API v3 returns the full size description (eg.: '1X', as opposed to '1').
ps:resize - add PX to the usage example
use APIv3 for ps:resize
heroku run: allow size to be anything, validation is in the server side
heroku ps: use APIv3
Merge branch 'master' into px-dynos

Conflicts:
	Gemfile.lock
one arg per line
damn misconfigured editor
Merge pull request #1010 from heroku/px-dynos

support for PX dynos