lock keras version to the latest version with cntk support

[ahmadalli]

https://github.com/keras-team/keras#multi-backend-keras-and-tfkeras

[opbld33]

Docs Build status updates of commit 5568161:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
articles/Using-CNTK-with-Keras.md	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching in the Docs contributor and Admin Guide
• See the frequently asked questions
• Post your question in the Docs support channel