gosrc: update Go version regexp for golang.org pages
gosrc: update Go version regexp for golang.org pages

The golang.org website has changed its UI design, and no longer
includes "Build version <version>" text at the bottom of each page.

The build version is still specified on each page in a short script.
Use that instead.

Updates golang/gddo#644

Change-Id: Iedac4fa1f44510af6462f9bb3431c5c23eaaed16
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/193258
Reviewed-by: Tuo Shan <e38988508c7dd9b59ee1f5735040284711960df8@google.com>

golang/gddo,golang/gddo,golang/gddo,golang/gddo