Make the next release version compliant with SemVer

As pointed out by Curtis Rueden, http://semver.org/ suggests a dash
between the 'rc' and the release candidate numer.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Start using the gpg-agent for deploying already

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
[maven-release-plugin] prepare release nar-maven-plugin-3.0.0-rc-2