Seems like the force option doesn't exist anymore.
There are now 4 of them:

force_checkout
: False
When checking out the local branch, the state will fail if there are unwritten changes. Set this argument to True to discard unwritten changes when checking out.

force_clone
: False
If the target directory exists and is not a git repository, then this state will fail. Set this argument to True to remove the contents of the target directory and clone the repo into it.

force_fetch
: False

If a fetch needs to be performed, non-fast-forward fetches will cause this state to fail. Set this argument to True to force the fetch even if it is a non-fast-forward update.

New in version 2015.8.0.

force_reset
: False
If the update is not a fast-forward, this state will fail. Set this argument to True to force a hard-reset to the remote revision in these cases.

I don't know which one this option was set for. In the meantime, in my case setting it to False worked, hence this PR.