[maven-release-plugin] prepare branch 1.8

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1458422 13f79535-47bb-0310-9956-ffa450edef68
[maven-release-plugin] prepare release 1.8

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1458424 13f79535-47bb-0310-9956-ffa450edef68