Deprecated packages still required in Dockerfile

[cynddl]

Hello,

It seems like Postgres 9.5 (as required in Gargamelle Docker file) is not available anymore in the repositories of Debian Stretch. Updating the Docker file to Postgres 9.6 solves the issue and does not seem to impact the deployment of the project.

Same for libgfortran-5-dev not available in Stretch anymore.