sandbox: update to go1.7rc6 (go1.7 final will come later)

Change-Id: I4658bd5e8b2a98124f6b68c867b0211c767f196c
Reviewed-on: https://go-review.googlesource.com/27077
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/sandbox/Dockerfile b/sandbox/Dockerfile
index 2173f2c..4cb00d6 100644
--- a/sandbox/Dockerfile
+++ b/sandbox/Dockerfile
@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-FROM golang:1.6
+FROM golang:1.7
 
 RUN apt-get update && apt-get install -yq --no-install-recommends patch bzip2