Merge remote-tracking branch 'remotes/stsquad/tags/pull-travis-20160621-1' into staging

This pull request contains:

  - disable sparse testing
  - add trusty build target
  - add libnfs-dev for NFS block driver

These are the same patches posted last week for any last minute review.

# gpg: Signature made Tue 21 Jun 2016 10:06:34 BST
# gpg:                using RSA key 0xFBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>"
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-travis-20160621-1:
  .travis.yml: disable Sparse testing
  .travis.yml: add trusty GCE target
  .travis.yml: add libnfs-dev for NFS block driver

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>