Bump parent POM version.

According to Oliver Giles in a Jenkins wiki page comment, a newer JDK
requires a newer version of org.jenkins-ci.plugins:
https://wiki.jenkins-ci.org/display/JENKINS/Plugin+tutorial?focusedCommentId=58002463#comment-58002463
According to Stephen Connolly in the jenkinsci-dev mailing list, 1.447
is a reasonable minimum version to use:
https://groups.google.com/d/msg/jenkinsci-dev/NQTVqUlWu1A/216FyBqycEgJ