resolvconf: Fix fetch error

Branch 'master' renamed to 'unstable', which causing following failure.

Error:
Fetcher failure: Unable to find revision cb19bbfbe7e52174332f68bf2f295b39d119fad3 in branch master even from upstream

Switch to 'unstanble' branch.

Signed-off-by: Naveen Saini <naveen.kumar.saini@intel.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>