[DCOS-40629] Bump and add useful packages

[vishnu2kmohan]

Added:

• nb2kg
• jupyter_enterprise_gateway
• jupyter_kernel_gateway
• pgcontents
• pydotplus
• s3contents

Updated:

• JupyterLab to 0.34.0
• MLFlow to 0.5.0
• Apache Toree to 0.2.0-incubating, which is the latest release version, no more -rc :tada:

Note: The TensorFlow 1.10.0 pip (wheel) package has a broken dependency on an older version numpy:

tensorflow 1.10.0 has requirement numpy<=1.14.5,>=1.13.3, but you'll have numpy 1.15.0 which is incompatible.

TensorFlow 1.9.0 however, works just fine with numpy 1.15.0 AFAICT :man_shrugging: