sshfs: switch to github

The project moved to github, so point the homepage and download URL
there.

[Thomas:
  - Remove comment "Switch to github helper on bump" since we clearly
    don't want to switch to the github helper if they continue to
    upload release tarballs.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>