Bug 1236592 - Heroku: Update pylibmc from v1.4.2 to v1.5.0

One of the most notable changes is a fix for a few memory leaks.

http://sendapatch.se/projects/pylibmc/changelog.html
https://github.com/lericson/pylibmc/compare/1.4.2...1.5.0

requirements.txt

@@ -5,7 +5,7 @@
 # Pylibmc must be in this file so that heroku knows it has to install
 # libmemcached when bootstrapping the containers.
 # More info here: https://devcenter.heroku.com/articles/memcachier#python
-pylibmc==1.4.2
+pylibmc==1.5.0
 
 django-pylibmc==0.5.0
 django-heroku-memcacheify==0.8