Make sure to remove 'install' directory if wallabag is already installed

Subsequent runs of wallabag's playbook result in the 'install' directory being
recreated, (after the "Clone wallabag" step) kicking wallabag's first time
setup procedure.

With this patch we automatically remove the 'install' directory if wallabag's
configuration file appears to be in place before cloning the repository.
Merge pull request #271 from lvillani/wallabag-dont-reinstall-on-upgrade

Merge pull request #271