BUG: fix kdeplot to allow use of fill=True after seaborn updates

[nicholas-ys-tan]

Resolves #289

@ResidentMario , not sure if you are still active in github, but would like to have a chat on whether you have long-term plans for this repo. In the mean time, I'll try submit PRs so geoplot is compatible with the latest versions of its dependencies and will commit them on my fork.