Revert "Switch to Apache II License", restore GPLv3+

This reverts commit 21d31238dba33e2441a1798e0569495389ab9cb5 and sets the
licence back to GPLv3+ to match Foreman.

puppetdb_foreman is likely a derivative work according to the FSF
interpretation: http://www.gnu.org/licenses/gpl-faq.html#GPLAndPlugins

It interacts with specific Foreman APIs (plugin registration, menus, RBAC),
shares Foreman models (hosts, settings), hooks (e.g. after_build), and only
makes sense when combined with Foreman.