updates to scikit-fmm cause errors with poreture

[npmitchell]

Updated scikit-fmm now returns an error for the kline_vessel demo:

File "test_vessel.py", line 22, in
[extracted_centerline, final_march] = kline_vessel(a, root_location, min_branch_to_root=mbtr_to_pass)
File "/path_to_code/poreture-master/poreture/extract_centerline.py", line 185, in kline_vessel
weighted_travel_time = skfmm.travel_time(phi, weighted_speed)
File "/path_to_python/python2.7/site-packages/skfmm/pfmm.py", line 173, in travel_time
int(self_test), TRAVEL_TIME, order, narrow, periodic)
RuntimeError: Negative discriminant in time marcher quadratic.

[starryevening]

scikit-fmm version 0.0.8 works