Add option to svnadmin to create pre 1.6 compatible repositories

This change is necessary to handle svn client (warnings) regarding the repository format:

    post commit FS processing had error:
    couldn't open rep-cache database
 
svnadmin man page:

--pre-1.6-compatible
  When creating a new repository, use a format that is compatible with
  versions of Subversion earlier than Subversion 1.6.