Install libffi-dev on frontend-app machines

To get around issues with SNI in python we are using pyOpenSSL to do
SSL, this requires a crypto library that uses libffi. My
performanceplatform-admin commit for reference:

https://github.com/alphagov/performanceplatform-admin/commit/56b8491fd27b19da38a8e062b0771e933ea2925d