travis: explicitly include gnutls to ensure it is updated

Travis includes gnutls in the default package set, but it is
an outdated version linkng to an incompatible libffi version.
The 'update: true' stanza causes the brew toolchain to be
updated but not the installed formula. It is possible to
run 'brew upgrade' to update installed formula, but this is
very slow adding more than 5 minutes to the build time.
Listing the gnutls package explicitly causes it to be updated
without extending the build time.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>