Remove v1.0.4 tag/version and update Changelog

[dregad]

The v1.0.4 tag is confusing, as earlier versions of the plugin were 1.2.x / 1.3.x, making it look like the version numbering went backwards... I think it should be deleted.

Likewise, the CHANGELOG.md only reflects the history since the Mantis 2.x compatibility update. You should remove 1.x references from there, and add history from the original README file.