security: Avoid using OpenSSL when using Go 1.6

As per https://go-review.googlesource.com/#/c/8968/, the optimized
amd64 assembly implementation for signatures and verification will
be part of Go 1.6, so OpenSSL and this cgo dependency will not be
required.

Change-Id: I7d23bff681c0032074846b3b322d95c3b3bc9a60
3 files changed