sandbox: update to go1.8

Change-Id: If17488a1f7e11a16962486d430192204cd4ce381
Reviewed-on: https://go-review.googlesource.com/37134
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/sandbox/Dockerfile b/sandbox/Dockerfile
index bdb6dc7..38bac72 100644
--- a/sandbox/Dockerfile
+++ b/sandbox/Dockerfile
@@ -7,7 +7,7 @@
 ENV GOPATH /go
 ENV PATH /usr/local/go/bin:$GOPATH/bin:$PATH
 ENV GOROOT_BOOTSTRAP /usr/local/gobootstrap
-ENV GO_VERSION 1.7.4
+ENV GO_VERSION 1.8
 
 # Fake time
 COPY enable-fake-time.patch /usr/local/sandbox/