Require CMake 3.x

We have several twiddly bits in our CMakeLists to work around building with both CMake 2.x and CMake 3.x. It'd be better and easier to maintain if we consolidated on a single version across all platforms.

Steps:

1. Upgrade all build slaves to install common version - many Linux distros don't have anything newer than 2.8.x in the repositories, so we will install from binaries.

2. Update top-level CMakeLists.txt to require this version.

3. Remove the various CMAKE_MAJOR_VERSION checks throughout the code.