Bump the development Ruby version

Update gem-signing cert

Adding release checksum.

Signing v0.23.0

Added tag v0.23.0 for changeset d2f51aec344e

Bump the minor version, update history

Fix breaking change in git gem

Update gem-signing cert, dependencies

Adding release checksum.

Signing v0.22.0

Added tag v0.22.0 for changeset 66dc3cb02fc4

Bump the minor version, update history and gemspec.

Make Sixfish the default RDoc generator

Fix diffing for the history file under Git

Adding release checksum.

Signing v0.21.0

Added tag v0.21.0 for changeset ce3ec0e563c1

Bump the minor version, update history.

Allow declaration of alternative RDoc generator

Only include plain files in the manifest