Fixed matplotlib-related tests Travis CI doesn't have X server.
Fixed matplotlib-related tests
Travis CI doesn't have X server.

anhiga/poliastro,anhiga/poliastro,Juanlu001/poliastro,newlawrence/poliastro,newlawrence/poliastro,newlawrence/poliastro,poliastro/poliastro,Juanlu001/poliastro,Juanlu001/poliastro,anhiga/poliastro