Bug 1160561 - Stop setting BROKER_URL to CLOUDAMQP_URL on Heroku
Bug 1160561 - Stop setting BROKER_URL to CLOUDAMQP_URL on Heroku

Since `BROKER_URL` on Heroku is now set to `$CLOUDAMQP_URL` and so
django-environ's proxied environment variables feature will fetch the
value from `CLOUDAMQP_URL` for us.

eg:
[~/src/treeherder]$ heroku run python
...
>>> import os
>>> import environ
>>> os.environ['BROKER_URL']
'$CLOUDAMQP_URL'
>>> env = environ.Env()
>>> env('BROKER_URL')
'amqp://REDACTED:REDACTED@REDACTED.rmq.cloudamqp.com/REDACTED'

wlach/treeherder,glenn124f/treeherder,KWierso/treeherder,akhileshpillai/treeherder,kapy2010/treeherder,tojon/treeherder,tojonmz/treeherder,KWierso/treeherder,glenn124f/treeherder,wlach/treeherder,KWierso/treeherder,akhileshpillai/treeherder,tojonmz/treeherder,kapy2010/treeherder,edmorley/treeherder,tojon/treeherder,tojon/treeherder,KWierso/treeherder,tojonmz/treeherder,glenn124f/treeherder,edmorley/treeherder,edmorley/treeherder,glenn124f/treeherder,akhileshpillai/treeherder,wlach/treeherder,akhileshpillai/treeherder,wlach/treeherder,kapy2010/treeherder,akhileshpillai/treeherder,glenn124f/treeherder,wlach/treeherder,kapy2010/treeherder,glenn124f/treeherder,tojonmz/treeherder,edmorley/treeherder,tojon/treeherder,kapy2010/treeherder,tojonmz/treeherder,tojonmz/treeherder,akhileshpillai/treeherder,wlach/treeherder