Upgrade Go to 1.7.6 / 1.8.3

Minor release notes: https://golang.org/doc/devel/release.html#go1.7 https://golang.org/doc/devel/release.html#go1.8.minor

This also should include: https://github.com/golang/go/issues/20040

Certain parts of our code do use P-256, but it's probably not exploitable. Updating is still a good idea.