Minimum _required_ version is only 3.0

While we do want the build server to be using the latest version of CMake, there is nothing specifically requiring that we mandate users make use of version 3.1 as well.  Thus, roll 3.0 is the minimum required, while 3.1 is merely the preference.
Merge pull request #304 from leapmotion/fix-cmakemin

Minimum _required_ version is only 3.0