release MeTA v1.3.8

Bug fixes

• Fix issue with confusion_matrix where precision and recall values were
  swapped. Thanks to @HusseinHazimeh for finding this!

Enhancements

• Better unit tests for confusion_matrix
• Add functions to confusion_matrix to directly access precision, recall, and
  F1 score
• Create a predicted_label opaque identifier to emphasize class_labels that
  are output from some model (and thus shouldn't be interchangeable)