Reduce default processor count to two under CI environments

Most CI implementations like travis / circleci run the builds
containerised. This can lead to the situartion where many more
processors are detected via /proc/cpuinfo and friends than are actually
usable through resource limits enforced by the containerization.
This can result in high resource consumption that might
result in abnormal build termination.

The default can easily be overriden in your CI setup.

* http://fabiokung.com/2014/03/13/memory-inside-linux-containers/