* Notify users of new versions of cellxgene
Fixes https://github.com/chanzuckerberg/cellxgene/issues/683
* Do not use PyGithub client
* Protect against AttributeError
* Document that all version tags must follow SemVer
* Release tags `should -> MUST` follow semantic versioning
* Add --no-cache-dir to make release-install target
Prevents installing from cache so you get the freshest release
* Testing releases is not optional
* Updated release documentation
