Merge remote-tracking branch 'remotes/stsquad/tags/travis-pull-05042016' into staging

This pull request includes:
  - further collapse of the build matrix
  - enabling MacOSX in the build
  - make -j3 change

Other pending updates are deferred for later in the cycle.

# gpg: Signature made Tue 05 Apr 2016 10:11:25 BST using RSA key ID 5A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>"

* remotes/stsquad/tags/travis-pull-05042016:
  .travis.yml: make -j3
  .travis.yml: enable OSX builds
  .travis.yml: collapse the test matrix

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