[maven-release-plugin] prepare release 1.8.1

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/branches/1.8@1465401 13f79535-47bb-0310-9956-ffa450edef68
[maven-release-plugin] prepare for next development iteration

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/branches/1.8@1465403 13f79535-47bb-0310-9956-ffa450edef68