Require clang 3.6

Require clang 3.6

This version is the lowest that supports a smooth c++14 debug experience.
The clang in XCode 6.3 is based off this branch, and that is also the
earliest that supported c++14.

Adjusted build matrices because clang 3.6.2 is still affected by AVX
codegen issues in release and debug mode. clang 3.7 is fine. Thus
moved some minor coverage to the post-submit matrix.

Updated install guide text also for some unrelated compiler-support
things.

Also removed an overlooked gcc version check

Refs #2831

Change-Id: Id780ff06147e54c7a4516cc33c9897a4a4d2d3fa