You can bump a formula entirely on the command line.
$ brew bump-formula-pr --strict faas-cli --tag=0.4.6 --revision=39183ee1b344a7bb8825bd2ebedce5c0bf4262c4
You'll be prompted to allow access to your git credentials in the OSX keychain. There may be some extra setup required tho, possibly setting up the remote?
Already up-to-date.
==> replace "0.4.5" with "0.4.6"
==> replace "e9c738f9a2833980a1bfc0102b3dfdf0cce2684a" with "39183ee1b344a7bb8825bd2ebedce5c0bf4262c4"
==> Downloading https://homebrew.bintray.com/bottles/hub-2.2.9.sierra.bottle.tar.gz