Switch to the official Pyrolite release from the one published under `org.spark-project`. Thanks irmen for making the releases on Maven Central. We didn't upgrade to 4.6 because we don't have enough time for QA. I excludes `serpent` from its dependencies because we don't use it in Spark.
~~~
[info]   +-net.jpountz.lz4:lz4:1.3.0
[info]   +-net.razorvine:pyrolite:4.4
[info]   +-net.sf.py4j:py4j:0.8.2.1
~~~

davies

Author: Xiangrui Meng <meng@databricks.com>

Closes #6472 from mengxr/SPARK-7926 and squashes the following commits:

7b3c6bf [Xiangrui Meng] use the official Pyrolite release