[TravisCI] - build protobuf3 GA

[lukeyeager]

Cherry-pick of NVIDIA#222

Clear the build cache and you'll start seeing failures like this in the BVLC fork:

[gtest-1.7.0.zip]
  End-of-central-directory signature not found.  Either this file is not
  a zipfile, or it constitutes one disk of a multi-part archive.  In the
  latter case the central directory and zipfile comment will be found on
  the last disk(s) of this archive.
unzip:  cannot find zipfile directory in one of gtest-1.7.0.zip or
        gtest-1.7.0.zip.zip, and cannot find gtest-1.7.0.zip.ZIP, period.

e.g.
https://travis-ci.org/BVLC/caffe/jobs/155609508 (that one failed because he based his PR off of an old commit without build caching)
https://travis-ci.org/NVIDIA/caffe/jobs/155406776

[shelhamer]

Thanks for the build care @lukeyeager!