Buildr uses Rake to do task dependency evaluation and execution. However, Rake doesn't have any knowledge of how to build Java projects specifically. Rather than re-writing every task it supports in ruby, Buildr uses ant tasks to implement some of its own tasks. (One I can think of off hand is running JUnit tests; I'm sure there are others.) So, Buildr needs Ant.
You can either add the appropriate remote repository or ensure that the correct version of the ant jar is in the m2 cache on every machine that will use your buildfile. For the artifact in your error message, the expected path would be $HOME/.m2/repository/org/apache/ant/ant/1.8.3/ant-1.8.3.jar
.
Note that ant is likely not the only dependency that buildr will attempt to download. You'll have to do this manual resolution for each dependency if you don't want to have a remote repository defined.
(Note that if you run buildr in a directory without a buildfile, it will offer to generate one for you. The one it generates will have an appropriate remote repository set up.)