Example release v1.2.4

This release is for an example command to see how this library works.

How to try out:

1. Install example command by go get -u github.com/rhysd/go-github-selfupdate/cmd/selfupdate-example
2. Check the version is v1.2.3 by running selfupdate-example -version
3. Run selfupdate-example -selfupdate and it will replace itself.
4. Check the version is v1.2.4 by running selfupdate-example -version