For Travis, move from whitelist to blacklist for packages in xs
For Travis, move from whitelist to blacklist for packages in xs

This should make adding packages in xs/ easier as now we will try to
compile them on Travis by default.

Signed-off-by: Christian Lindig <4f88b50bc9dba63c9a7d9e208d4b3ac6f1a52800@citrix.com>

xapi-project/xs-opam,lindig/xs-opam,mseri/xs-opam,gaborigloi/xs-opam,mseri/xs-opam