Bump the minimum required version of Go to 1.7. This removes version specific code and makes sure users have the latest mobile related fixes to Go applied. Also, this change is necessary when runtime.KeepAlive is introduced in a later CL. Change-Id: I8441a28aef7f645379fbd8f00edabe3c3fb219de Reviewed-on: https://go-review.googlesource.com/35953 Reviewed-by: David Crawshaw <crawshaw@golang.org>