Add poliastro to environment.yml dependencies
Add poliastro to environment.yml dependencies

This should help making the examples on mybinder.org directly runnable.
anhiga/poliastro,poliastro/poliastro,anhiga/poliastro,newlawrence/poliastro,Juanlu001/poliastro,newlawrence/poliastro,anhiga/poliastro,newlawrence/poliastro,Juanlu001/poliastro,Juanlu001/poliastro