SPARK-1121 Only add avro if the build is for Hadoop 0.23.X and SPARK_YARN is set

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #6 from ScrapCodes/SPARK-1121/avro-dep-fix and squashes the following commits:

9b29e34 [Prashant Sharma] Review feedback on PR
46ed2ad [Prashant Sharma] SPARK-1121-Only add avro if the build is for Hadoop 0.23.X and SPARK_YARN is set