Cherry-pick "Merge branch 'CMakeVersionUpdates-1.0-20181009' into '1.0'" into master

CMake Version Updates, 1.0 branch (2018.10.09.)

See merge request !360 (merged)

(cherry picked from commit cea14a3f)

• a5591ccb Upgraded all projects to require at least CMake version 3.6.
• 47010aad Setting the minimum CMake version required for the HDF5 build explicitly.
• 938e64d8 Added a CMake version requirement to AtlasCMake itself.