You'll need to add a RankLib <dependency> tag set to your existing <dependencies> list.
<dependency>
<groupId> edu.umass.ciir </groupId>
<artifactId> RankLib </artifactId>
<version> 2.6 </version>
</dependency>
Download the desired RankLib version jar and manually install it in your maven
(.m2) repository.
mvn install:install-file -DgroupId=edu.umass.ciir -DartifactId=RankLib -Dversion=2.6 -Dpackaging=jar -Dfile=/path/to/downloaded/RankLib-2.6.jar
There are apparently ways to have maven directly download the jar from SourceForge, but
I am not clear on the method. It invovles a <repository> tag set with <id> and <url>
definitions.
If you want the latest RankLib SNAPSHOT you can download the sources from SourceForge
and do a "mvn install" which will compile, package and install the sources for you.