Replace deprecated SQLContext with SparkSession for DataFrame / DataSet / RDD generators

[AndyLeGrand]

E.g. the DataframeGenerator.arbitraryDataFrame() method takes a SQLContext as parameter. According to the Spark docs [1], this has been deprecated since Spark 2.0 in favor of SparkSession. Would be nice to replace this.

[1] https://spark.apache.org/docs/latest/api/java/org/apache/spark/sql/SQLContext.html