Keras-mxnet 2.2.4 release

[roywei]

Summary

• Pull all changes from keras-team/keras 2.2.4 (commit d059890d0342955e968fdf97b5a90d19c9d68b4e)
• fixed conflicts
• resolved failing unit tests and style checks

Related Issues

PR Overview

• [ ] This PR requires new unit tests [y/n] (make sure tests are included)
• [ ] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [ ] This PR is backwards compatible [y/n]
• [ ] This PR changes the current API [y/n]

[kalyc]

Can you add link to the pickle issue that is opened with keras-team for mxnet backend?

[roywei]

@kalyc it been tracked here:https://github.com/awslabs/keras-apache-mxnet/issues/194