Fixed .gitmodule for Ansible deployments

Currently, Ansible always set the --remote got git submodule update operations.
Since no branch is specified for the submodule, it defaults to origin/master which gives an error (there is no master branch in [elFinder](https://github.com/laclasse-com/elFinder).

This patch adds a branch name for the submodule, so deploys can run fine. A request has been sent to the ansible ML to confirm this is an issue on the ansible side.