The buildnumber plugin will access your working copy to get the revision and since you use TortoiseSVN 1.8.x your working copy is in svn 1.8.x format. Hence svnkit which is used by the buildnumber plugin needs to support svn 1.8.x as well, which the version you're using (1.7.4-v1) most probably doesn't.
You thus need a newer version of svnkit, e.g.
<dependency>
<groupId>org.tmatesoft.svnkit</groupId>
<artifactId>svnkit</artifactId>
<version>1.8.3-1</version>
</dependency>