Prepare for the 1.5.4 tag


git-svn-id: https://svn.apache.org/repos/asf/apr/apr-util/branches/1.5.x@1625429 13f79535-47bb-0310-9956-ffa450edef68
1.5.4 is tagged; bump to 1.5.5 (development)


git-svn-id: https://svn.apache.org/repos/asf/apr/apr-util/branches/1.5.x@1625431 13f79535-47bb-0310-9956-ffa450edef68