Bumping minimum CMake requirement

Hey all,

At the dev meeting two weeks ago there was discussion about bumping the
minimum required CMake version for the project to 3.x from 2.8.11. As far
as we can tell, a minimum version of 3.5 is reasonable and easy to obtain
for every system we support including Debian and older releases of Ubuntu.
On Raspberry Pi I'm able to get 3.7.2 without any extra effort.

This would be nice for a few reasons:
- the Ableton Link PR is going to bump the requirement to 3.0 anyway
- means we don't have to support older buggy versions of CMake or
deprecated behavior
- removes need to have separate requirements between supernova and the rest
of the project

So, does anyone have any issues with 3.5 as a minimum? If not I'll open a
PR sometime within the next week. This will be for the main develop branch.

-Brian