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.
I confess I'm a bit lost here.  What is the reason for saying `github.com/getlantern/flashlight` and not `github.com/getlantern/flashlight-build`?