New version: 2.1.51-1

This merge is bumping targetcli-fb from 2.1.fb49-1 to 1:2.1.51-1 because upstream removed "fb" from their versioning.

I am updating Ubuntu before our freeze for 20.04 and wanted to propose the version we're including in 20.04 for merge.

Thanks for considering this merge!

Note: I also changed debian/watch.

$ sudo dpkg -i ./targetcli-fb_2.1.51-1_all.deb
(Reading database ... 43932 files and directories currently installed.)
Preparing to unpack .../targetcli-fb_2.1.51-1_all.deb ...
Unpacking targetcli-fb (1:2.1.51-1) over (2.1.fb49-1) ...
Setting up targetcli-fb (1:2.1.51-1) ...
Processing triggers for man-db (2.9.1-1) ...