Replace 'which' with 'command -v'

Replace 'which' with 'command -v'

The 'which' command is deprecated on Debian Sid as it is not POSIX
compliant and it's behavior is therefor not consistent, so replace it
with 'command -v' which is POSIX compliant.
See https://stackoverflow.com/a/677212 for details.

Also replaced a use of backticks (`) as that is deprecated as well.
See https://github.com/koalaman/shellcheck/wiki/SC2006 for details.

Signed-off-by: Diederik de Haas <didi.debian@cknow.org>