Pin requirements to the known good set to avoid potential future issues

requirements/base.txt

@@ -1,5 +1,7 @@
-Django
-elasticsearch
-wagtail
-wagtailfontawesome
-Pillow
+Django==1.10.5
+# http://docs.wagtail.io/en/v1.8.1/topics/search/backends.html#elasticsearch-backend
+# Not utilized by default; uncomment and review the above document
+# elasticsearch==5.1.0
+wagtail==1.8.1
+wagtailfontawesome==1.0.6
+Pillow==4.0.0

requirements/heroku.txt

@@ -1,6 +1,6 @@
 -r base.txt
 # Additional dependencies for Heroku deployment
-dj-database-url
-gunicorn
-psycopg2
-whitenoise
+dj-database-url==0.4.1
+gunicorn==19.6.0
+psycopg2==2.6.2
+whitenoise==3.2.2