• Add support for SparkDFDataset and caching (HUGE work from @cselig)
• Migrate distributional expectations to new testing framework
• Add support for two new expectations: expect_column_distinct_values_to_contain_set
  and expect_column_distinct_values_to_equal_set (thanks @RoyalTS)
• FUTURE BREAKING CHANGE: The new cache mechanism for Datasets,
  when enabled, causes GE to assume that dataset does not change between evaluation of individual expectations.
  We anticipate this will become the future default behavior.
• BREAKING CHANGE: Drop official support for python 2 and pandas < 0.22