xen-gnt now needs oasis

Upstream is trying to avoid checking in autogenerated rubbish, hence
the dep.
mirage-xen: add depext on pkg-config for ubuntu

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
travis: update packages after adding remote
Merge pull request #24 from djs55/update-deps

Improve xen-gnt, mirage-xen
mirage-skeleton: clone the latest master branch in travis