The travis-ci environment is based on Ubuntu 12.04 LTS, which has an
older protobuf version (2.4.1). We need to depend on features that are
only available in 2.5.0 (and, later, 2.6.0), so instead of satisfying
the protobuf build dependencies from the Ubuntu repository, download and
install our own copy of protobuf.

The travis-ci build from this commit should succeed, since we are
compatible with protobuf 2.5.0.

Based on a patch from Ilya Lipnitskiy.