cbdeps: Support Debian numbering

Identify Debian using it's major version (i.e. ignore the minor version).

Change-Id: I9aa4ac04e3451937a3eca3e7c80cf247ef4bca5a
Reviewed-on: http://review.couchbase.org/43799
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>