Improved dependency upgrade handling

[freeman-lab]

Consider using trick from seaborn (and statsmodel) for only adding dependencies to requirements if they cannot be imported, to prevent unintended upgrades of dependencies.

see e.g. https://github.com/mwaskom/seaborn/blob/master/setup.py

thanks @mwaskom for the suggestion!

[mwaskom]

For posterity, the original pip issue: pypa/pip#59, which remains open.