Update SparkContextFunctions.scala

To make it Serializable
take a look at this http://stackoverflow.com/questions/23737804/enriching-sparkcontext-without-incurring-in-serialization-issues
Merge branch 'b1.0' of github.com:datastax/spark-cassandra-connector into b1.1
Made SparkContext variable transient in SparkContextFunctions (PR #373 by @critikaled)
Merge branch 'b1.0' of github.com:datastax/spark-cassandra-connector into b1.1
Merge branch 'b1.1' of github.com:datastax/spark-cassandra-connector