If you have upgraded WPScan to version 2.2 with this command:
$ sudo wpscan --updateit is likely that you have this problem:
You have requested:
rspec-mocks <= 2.14.2
The bundle currently has rspec-mocks locked at 2.14.3.
Try running `bundle update rspec-mocks`
Run `bundle install` to install missing gems
The resolution is really easy, follow this command:
$ cd /opt/backbox/wpscan/
$ bundle update rspec-mocks
$ bundle install
Good Job

Andrea