Support Notebook 6

[ssanderson]

#65

[devstein]

@ssanderson Is there any timeline for this?

[devstein]

For those looking to only use HybridContentManager and need support for all notebook versions, we created a hybridcontents fork

[ssanderson]

Fixed as of pgcontents 0.6: https://github.com/quantopian/pgcontents/releases/tag/v0.6.0