That will be the go version at compile time. So read $GOROOT/VERSION, or
fall back to executing `$GOROOT/bin/go version` to find the go version
currently in GOROOT.
Test: Ensure everything rebuilds when switching between go1.8rc2 and go1.8
Change-Id: I8738a7aa249a088b1e0668af260fa3974844dab7