Bug 1160561 - Define IS_HEROKU in the dyno profile rather than Heroku env
Bug 1160561 - Define IS_HEROKU in the dyno profile rather than Heroku env

`IS_HEROKU` isn't something that will differ between stage/prod, and is
not going to change any time soon. So let's just get post_compile to
add it to the dyno profile script, so it's one less variable to have to
remember to set via the Heroku CLI/dashboard.

Quoting from:
http://blog.doismellburning.co.uk/2014/10/06/twelve-factor-config-misunderstandings-and-advice/

"12factor says your applications should read their config from the
environment; it has very little to say about how you populate the
environment ? use whatever works for you".

tojon/treeherder,kapy2010/treeherder,gbrmachado/treeherder,KWierso/treeherder,adusca/treeherder,gbrmachado/treeherder,jgraham/treeherder,avih/treeherder,sylvestre/treeherder,tojonmz/treeherder,sylvestre/treeherder,avih/treeherder,parkouss/treeherder,kapy2010/treeherder,deathping1994/treeherder,rail/treeherder,jgraham/treeherder,deathping1994/treeherder,kapy2010/treeherder,jgraham/treeherder,adusca/treeherder,akhileshpillai/treeherder,edmorley/treeherder,avih/treeherder,glenn124f/treeherder,rail/treeherder,glenn124f/treeherder,KWierso/treeherder,moijes12/treeherder,parkouss/treeherder,adusca/treeherder,adusca/treeherder,adusca/treeherder,jgraham/treeherder,deathping1994/treeherder,tojonmz/treeherder,moijes12/treeherder,wlach/treeherder,glenn124f/treeherder,akhileshpillai/treeherder,deathping1994/treeherder,moijes12/treeherder,parkouss/treeherder,KWierso/treeherder,tojonmz/treeherder,wlach/treeherder,moijes12/treeherder,akhileshpillai/treeherder,wlach/treeherder,tojonmz/treeherder,edmorley/treeherder,tojonmz/treeherder,akhileshpillai/treeherder,wlach/treeherder,tojon/treeherder,gbrmachado/treeherder,avih/treeherder,glenn124f/treeherder,gbrmachado/treeherder,avih/treeherder,glenn124f/treeherder,wlach/treeherder,jgraham/treeherder,sylvestre/treeherder,wlach/treeherder,KWierso/treeherder,tojonmz/treeherder,rail/treeherder,tojon/treeherder,sylvestre/treeherder,deathping1994/treeherder,rail/treeherder,sylvestre/treeherder,kapy2010/treeherder,moijes12/treeherder,glenn124f/treeherder,rail/treeherder,akhileshpillai/treeherder,edmorley/treeherder,rail/treeherder,moijes12/treeherder,edmorley/treeherder,sylvestre/treeherder,parkouss/treeherder,gbrmachado/treeherder,parkouss/treeherder,avih/treeherder,gbrmachado/treeherder,tojon/treeherder,jgraham/treeherder,akhileshpillai/treeherder,deathping1994/treeherder,kapy2010/treeherder,parkouss/treeherder,adusca/treeherder