Update go to 1.7.4 and 1.6.4 to fix security issues (#2467)

* Update go to 1.7.4 and 1.6.4 to fix security issues

The go team recently rolled out two releases to address security
issues.  Details available on the [go release
site](https://golang.org/doc/devel/release.html).

This commit updates our explicitly supported versions.

It also includes a comment about two CentOS requirements (enable
user_namespace and ensure that the static c library is installed) that
are required for the pacakges to pass their tests.

* Flake8 cleanup