cmake: Bump required cmake version to 3.0
cmake: Bump required cmake version to 3.0

The introduced changes are not compatible with cmake 2.8.12, and we
weren't testing compatibility with that version anyway. Since we depend
on recent compilers (GCC-5) there's no harm in doing the same for cmake
(and 3.0 is not that young anyway - its been 2 years). This will reduce
maintanence burden going forward.

FlexCoreLib/flexcore,CasparKielwein/flexcore