Use Go 1.7.1 for building OSX client binary

For Sierra we need Go 1.7 at present, until 1.6.4 is out and there
is a matching Docker release.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

Dockerfile

@@ -120,7 +120,7 @@ RUN set -x \
 # IMPORTANT: If the version of Go is updated, the Windows to Linux CI machines
 #            will need updating, to avoid errors. Ping #docker-maintainers on IRC
 #            with a heads-up.
-ENV GO_VERSION 1.6.3
+ENV GO_VERSION 1.7.1
 RUN curl -fsSL "https://storage.googleapis.com/golang/go${GO_VERSION}.linux-amd64.tar.gz" \
 	| tar -xzC /usr/local