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".

bin/post_compile

@@ -16,6 +16,8 @@ source $BIN_DIR/utils
 # Only variables that are rarely changed and not site-specific should
 # be set here. Set everything else using Heroku's CLI / dashboard.
 
+set-env IS_HEROKU 1
+
 # Override the hostname that is displayed in New Relic, so the Dyno name
 # (eg "web.1") is shown, rather than "Dynamic Hostname". $DYNO is quoted
 # so that it's expanded at runtime on each dyno, rather than now.