add version flag based on VERSION file

There's a couple tools that embed static assets in go binaries: https://github.com/aleksa/gopacker, https://github.com/jteeuwen/go-bindata

All of them seemed a bit heavyweight for what we want to do here (basically just keep the `version` command line flag up to do date with the `VERSION` file, without requiring manual editing) but I could be convinced to try go-bindata or something else. gopacker is a perl script.