v6.5.0: Supervised similarity, fancier embedding and improvements to linear model

✨ Major features and improvements

• Improve GPU support.
• Add classes for siamese neural network architectures for supervised similarity.
• Add HashEmbed class, an embedding layer which uses the hashing trick to support a larger vocabulary in a shorter table.
• Add support for distinct feature columns in the Embed class.

🔴 Bug fixes

• Fix model averaging for linear model.
• Fix resume_training() method for linear model.
• Fix L1 penalty for linear model.

📖 Documentation and examples

• Add supervised similarity example for Quora, StackExchange and SNLI data.