doc: update suggested Go installation version

Previously the docs suggested downloading 1.7.2 (no good) or 1.7.3 (has
security flaw).

Change-Id: Ifabe85908df8d6591c0957ef4bb1b800fc145793
Reviewed-on: https://go-review.googlesource.com/33808Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/install-source.html

@@ -212,7 +212,7 @@ To build without <code>cgo</code>, set the environment variable
 Change to the directory that will be its parent
 and make sure the <code>go</code> directory does not exist.
 Then clone the repository and check out the latest release tag
-(<code class="versionTag">go1.7.2</code>, for example):</p>
+(<code class="versionTag">go1.7.4</code>, for example):</p>
 
 <pre>
 $ git clone https://go.googlesource.com/go
@@ -400,7 +400,7 @@ New releases are announced on the
 <a href="//groups.google.com/group/golang-announce">golang-announce</a>
 mailing list.
 Each announcement mentions the latest release tag, for instance,
-<code class="versionTag">go1.7.2</code>.
+<code class="versionTag">go1.7.4</code>.
 </p>
 
 <p>