support networkx 2.0+ compatibility

[ixcat]

As discovered in #361 and slack discussion,
the new networkx 2.0 release (2017-09-20) incorporates several API changes which cause regressions in datajoint-python.

https://networkx.github.io/documentation/stable/release/release_2.0.html
https://networkx.github.io/documentation/latest/release/migration_guide_from_1.x_to_2.0.html

This issue is to provide support for v2.0 -
Another one will be created to temporarily force v1.1x in the pip requirements until 2.0 support can be implemented.

[ixcat]

PR issued in #443