Only darwin/clang needs GTEST to supply its own tr1 tuple.

Review: https://reviews.apache.org/r/19077
Switched to cxx11 by default for libprocess.

Review: https://reviews.apache.org/r/18577
Switched to cxx11 by default for mesos.

Review: https://reviews.apache.org/r/18576
Fixed fetcher to properly fetch multiple URIs.

Review: https://reviews.apache.org/r/18974
Added test for subprocess with environment variable set.

Review: https://reviews.apache.org/r/19144
Updated CHANGELOG for 0.18.0-rc4.
Removal of python-boto from the repo and add configure checks.

Python boto exists on nearly all platforms that I can think of. It
makes sense to add the check to configure and remove from the repo.

Review: https://reviews.apache.org/r/18910
Fixed the path to 'mesos-master' in configuration.md.