Based on transformers v3.5.1

New

• New model with adapter support: DistilBERT (@calpt via #67)
• Save label->id mapping of the task together with the adapter prediction head (@hSterz via #75)
• Automatically set matching label->id mapping together with active prediction head (@hSterz via #81)
• Upgraded underlying transformers version (@calpt via #55, #72 and #85)
• Colab notebook tutorials showcasing all AdapterHub concepts (@calpt via #89)

Fixed

• Support for models with flexible heads in pipelines (@calpt via #80)
• Adapt input to models with flexible heads to static prediction heads input (@calpt via #90)