v0.5.0

This is a fairly minor update, but important for people creating new Klaxon instances on Heroku. Relatedly, we've upgraded to using Ruby 2.7.2. That is the reason for the minor version bump.

There's also a bug fix for an issue that may have been causing some manually entered CSS selectors to not function properly. So it's possible you might start seeing alerts from pages that weren't being properly monitored before.

Thanks to @captn3m0 for the docker patch, and to @harrislapiroff, and @tszumowski for their help in identifying and debugging ongoing Sendgrid issues, some of which is reflected in updates to the documentation.