Convert backslashes to slashes before passing path to zip.

Because the latter expects only slashes as separators.

Change-Id: Ic295cd02ac1786df5c643528de1af7d896d45d10
Reviewed-on: http://review.couchbase.org/40587
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>