Fix bug 1527507: Add requirements.txt file (#1225)

requirements.txt

@@ -0,0 +1,5 @@
+# This file is required by Heroku: "To specify Python package dependencies on Heroku via pip,
+# add a pip requirements file named requirements.txt to the root of your repository."
+#
+# Source: https://devcenter.heroku.com/articles/python-pip
+-r requirements/default.txt