Orbot version appears to be outdated
The latest version of org.torproject.android shipped by F-Droid is 15.0.1-RC-3-PIE (added on 2015-07-16) but according to the official repository at https://gitweb.torproject.org/orbot.git the latest tagged version is 15.1.2 from 3 days ago.
While the Guardian Project have their own F-Droid repository including Orbot, I suppose it would be ideal to have the current version in F-Droid as well. Importantly, the new version seemingly includes VPN support (making transparent proxying possible without needing root).