getlantern/lantern#2105 crosscompile now uses tag for latest commit id i...

...f available, else just uses latest commit id

This closes getlantern/lantern#2105 and also getlantern/lantern#2106.
Cool :)  For ultimately clarity, I'll just state it - this script builds flashlight.  flashlight-build isn't actually a Go project, it's just a container for a complete GOPATH that happens to include flashlight.