<groupId>org.apache.avro</groupId>
<artifactId>avro-mapred</artifactId>
<version>1.7.4</version>
<classifier>hadoop2</classifier>
the hadoop2 version is for cdh4. I think the fix is simply removing "hadoop2" from the project/build.scala file on the cdh3 branch:
     Seq(
      "org.apache.avro"          %  "avro-mapred"               % "1.7.4",
      "org.apache.avro"          %  "avro"                      % "1.7.4",
      "com.thoughtworks.xstream" %  "xstream"                   % "1.4.4"            intransitive(),
      "com.googlecode.kiama"     %% "kiama"                     % "1.5.0-SNAPSHOT",