version: add go1.8rc1
version: add go1.8rc1

Adding ahead of the release so it's ready to go once the binaries land
and it's announced.

Change-Id: I04a68f5a6a50ae7a22df9cba14005237a276ffaa
Reviewed-on: https://go-review.googlesource.com/34913
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build