Force go 1.7.1 in circle.yml to resolve failure to get golint

Signed-off-by: Alessandro Boch <aboch@docker.com>

libnetwork/circle.yml

@@ -1,4 +1,6 @@
 machine:
+  environment:
+    GODIST: "go1.7.1.linux-amd64.tar.gz"
   services:
     - docker