update 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?
`%%` automatically appends whatever version of scala you are using to the artifact id, so we should continue to use that instead of specifying it manually.

Also, remove the extra space between `+=` and `"`