Added nbformat as a dependency and used version boolean specifiers.

[moorepants]

Checklist

• [x] Used a fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [x] Re-rendered with the latest conda-smithy
• [x] Ensured the license file is being packaged.

The dependencies for Plotly 2.6.0 were not correct. Fixes #34

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[moorepants]

@conda-forge-admin please rerender

[moorepants]

@chohner This is ready to be reviewed.

[chohner]

Thank you!