env: add subversion to the linux-x86-stretch dockerfile

CL 114595 added git and hg to the stretch builder to fix cmd/go tests,
but I missed that one test is using svn. Add svn too.

Change-Id: I8c6555c1c71cecd4012136ca413008025deeef32
Reviewed-on: https://go-review.googlesource.com/114676
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/env/linux-x86-stretch/Dockerfile b/env/linux-x86-stretch/Dockerfile
index 8f15e5d..202ed74 100644
--- a/env/linux-x86-stretch/Dockerfile
+++ b/env/linux-x86-stretch/Dockerfile
@@ -37,6 +37,7 @@
 	openssh-server \
 	git \
 	mercurial \
+	subversion \
 	&& rm -rf /var/lib/apt/lists/*
 
 RUN mkdir -p /go1.4-amd64 \