Explicitly declare dependencies

[cchaudier]

As a follow-up to the issue #101 can you set the versions of the dependencies?
First of all borgbackup and borgmatic.

This is to prevent unexpected changes in cli.
This is a good practice promoted by the 12 factors.

I think you can do this here whit this code :

borg_python_packages:
  - borgbackup==1.2.2
  - borgmatic==1.7.1

[m3nu]

Unfortunately not because some OS use old Python versions and will pull in an older version.

[Sebclem]

I think this can be closed as #107 is a workaround