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.
Similarly, this script is not at the root of the repo where this README is, so it becomes `./src/github.com/getlantern/flashlight/tagandbuild.bash`.  Alternatively, we can ask the user to `cd src/github.com/getlantern/flashlight` first.