Update compiler requirements for C++14

[cnweaver]

Follow-up to #148.

[cozzyd]

those versions seem correct to me, based on docs. This might be slightly painful for EL7 systems that have gcc 4.8 as a default, but it seems reasonable to use a standard 10 years after it is adopted...

[arahlin]

those versions seem correct to me, based on docs. This might be slightly painful for EL7 systems that have gcc 4.8 as a default, but it seems reasonable to use a standard 10 years after it is adopted...

EL7 is EOL this year, so this is probably ok. We [SPT] also use clustertools (https://github.com/SouthPoleTelescope/clustertools) with more recent compilers on any EL7 systems anyway.

[arahlin]

I think I'm just going to merge this, it's just a README update that isn't going to set any fires.