update to the code to work with latest version of spark1.2

Hi, @marmbrus 
Yesterday has released the spark1.2. For the latest Spark SQL external api. 
I have made some change for the sql-avro project to work with the latest api.
1. modify the dependency of spark sql to the latest.
2. The trait `TableScan` is no longer a trait but a abstract class which extends BaseRelation.
So only `AvroRelation ` need to extend `TableScan`.

Any suggestions?