Update minimum required version of CMake to 3.8

[mmisono]

CMake 3.8 is required to use C++17.
cf. Kitware/CMake@ae1a681

closes #1193

[mmisono]

It seems Ubuntu 16.04 has CMake 3.5 by default. So users will need to update CMake to build.
alpine 3.8 has CMake 3.11.

[danobi]

I think @fbs brought up downgrading back to C++14 in IRC. Maybe we should do that for this coming release, then go back up to C++17 and add this change too.

What do you think?

Also, please come join the IRC channel :)

[fbs]

I think @fbs brought up downgrading back to C++14 in IRC. Maybe we should do that for this coming release, then go back up to C++17 and add this change too.

My idea was to do the C++17 right after the 0.9.5 release. That way we can make this the last 16.04 release instead of dropping it half way through without using any features of it yet (like we did now). They can still profit from any improvements that land in 0.9.5 but one that is release 16.04 will be ~4y old so dropping support sounds ok to me

I will add this to the next milestone so we can do it right after we've released

Also, please come join the IRC channel :)

+1