Merge remote-tracking branch 'remotes/stsquad/tags/pull-travis-20170112-1' into staging

A couple of fixes to reduce the matrix some more that just missed the
last iteration.

# gpg: Signature made Thu 12 Jan 2017 13:01:49 GMT
# 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-20170112-1:
  travis: add Trusty with clang stable build
  travis: trim out most clang builds

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