activations moved from keras.layers to keras in keras 2.2.0

[jose-carmona]

We used Keras-Transformer in Kaggle University of Liverpool - Ion Switching competition but we had an issue because activations wasn't found in keras_transformer/transformer.py

You can see the notebook in https://www.kaggle.com/josecarmona/test-of-transformer-over-data-without-drift

We check keras_transformer/extras.py was moved but not in keras_transformer/transformer.py so we fix it and increase requirement of Keras version to 2.2.0.