Drop support for networkx 1.x

[beckjake]

Feature

Drop support for networkx <2

Feature description

Currently we have this comment: https://github.com/fishtown-analytics/dbt/blob/42fb12027c6ccec4f1cd5b71391d469dd191581c/core/dbt/linker.py#L213

Which indicates to me that we should probably just drop 1.x support entirely and use 2.x exclusively, with its fixed nx.algorithms.find_cycle implementation.

Also, I am pretty sure we have zero interest in supporting old networkx versions, which we inevitably will otherwise.

Who will this benefit?

dbt maintainers!