.travis.yml: add libnfs-dev for NFS block driver

Let's ensure that block/nfs.o is built in Travis.

This patch depends on the following build fixes:
1. block/nfs: add missing #include "qapi/error.h"
2. block/nfs: add missing #include "qemu/cutils.h"

This patch also depends on Travis adding libnfs-dev to the list of
approved packages.  This patch can be safely committed but will not do
anything until the Travis maintainers allow libnfs-dev to be installed.
Please see the GitHub Issue I raised here:
https://github.com/travis-ci/apt-package-whitelist/issues/2788



Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>