appengine/internal: Split VersionID on "." instead of "_".
appengine/internal: Split VersionID on "." instead of "_".

On both Classic and Managed VM apps, the VersionID is returned
in the form of "X.Y". This fixes the background context which
requires only the major portion of the VersionID (the "X" part).

Change-Id: Ibc6aeca521be6ccdfab5369bde1e1c4bc1234897

mzimmerman/appengine,golang/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,mzimmerman/appengine,golang/appengine,flowlo/appengine,flowlo/appengine,mzimmerman/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,flowlo/appengine