Remove call to JRE-7-only library method
Make maven-gpg-plugin ignore detached sig files

This prevents the creation of worthless "foo.asc.asc" files when
building signed release artifacts multiple times in succession without
an intervening `mvn clean`.
Tweaks to RELEASING and gh-pages update script

Collected during the 0.4.2 release
Control maven-scm-plugin version (1.9)