Bug 1394593 - Heroku: Use 'DYNO' to check if running on Heroku

On Heroku, the environment variable `DYNO` is always set in the
environment. As such, we can just use that to determine whether
code is being run on Heroku, rather than having to add our own
environment variable.

bin/post_compile

@@ -23,8 +23,6 @@ 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.

treeherder/config/settings.py

@@ -631,7 +631,7 @@ KEY_PREFIX = TREEHERDER_MEMCACHED_KEY_PREFIX
 # This code handles the memcachier service on heroku.
 # TODO: Stop special-casing Heroku and use newer best practices from:
 # https://www.memcachier.com/documentation#django.
-if env.bool('IS_HEROKU', default=False):
+if 'DYNO' in env:
     # Prefs taken from:
     # https://github.com/rdegges/django-heroku-memcacheify/blob/v1.0.0/memcacheify.py#L30-L39
     CACHES['default'].update({