[RFC] Increase minimum required compiler versions

Currently, we claim to support `g++-4.8` and `clang-3.4` as minimum compiler versions (among others). [Googletest](https://github.com/google/googletest), on which we depend for testing (but not in general) increased their minimum support to `g++-5` and `clang-5`.

The oldest LTS version of ubuntu we want to support is trusty (14.04) which isn't EOL until April 2022. Out of [the box](https://packages.ubuntu.com/trusty/allpackages) you can get `g++-4.8` and `clang-3.6`.

I noticed this while trying to set up an [ubuntu 14.04 CI build](https://github.com/google/benchmark/pull/1131)

So we have a couple of options:

1. increase our minimum compiler versions to match googletest.
* Users on ubuntu 14.04 (or similar older platforms) would need to find sources for newer compiler versions.
* Tests/CI would continue to run against known good compiler versions
2. increase our minimum compiler version _for benchmark developers_ to match googletest.
* This would mean users on ubuntu 14.04 could still use the library.
* No tests on CI or developer machines would be testing those codepaths beyond building them.

I'd really like to do 2, but i'm not sure of the safety of not having runnable tests and maybe we should just jump straight to 1.

Thoughts?