Release 0.13.1

Major Features and Improvements

• Added support for squared pearson correlation (R squared) post export
  metric.

Bug fixes and other changes

• Cast / convert labels for precision / recall at K so that they work even if
  the label and the classes Tensors have different types, as long as the types
  are compatible.
• Post export metrics will now also search for prediction keys prefixed by
  metric_tag if it is specified.
• Added support for precision/recall @ k using canned estimators provided
  label vocab not used.
• Preserve unicode type of slice keys when serialising to and deserialising
  from disk, instead of always converting them to bytes.
• Use __slots__ in accumulators.

Breaking changes

• Expose Python 3 types in the code (this will break Python 2 compatibility)