Two little fixes for comfort

This patch includes two minor and very little fixes.

First, the default value for the "shibboleth_idp_version" variable is changed to the currently active version. At the moment the download is broken when using the example vars file. This patch ensures that it works, even when the user does not change the "shibboleth_idp_version" variable.

Second: Since GPG is required on the target system for download verification, we should include it in the software installation tasks. I added gpg to the list of packages, that will be installed when the "shibboleth_idp" role is running. At the moment the playbook fails on some apt-based systems. This fixes it.