Force update of repo2docker Docker image

[pierrepo]

With the current situation:

https://github.com/plasmabio/plasma/blob/d7d670309fbab871d77ae1bdb733e167cbf86fad/ansible/tljh.yml#L46-L51

The Docker image of repo2docker is never updated. According to Ansible documentation, we should add the parameter:

force_source: yes

to update repo2docker to the newest version.

@jtpio what do you think?