Fix compatibiility with matplotlib 1.1

[keflavich]

Currently the following failure happens:

xmin = -0.5, xmax = 0.5, ymin = -0.5, ymax = 0.5
x = [0.99950000000000006, 0.99950000000000006, 1.0004999999999999, 1.0004999999999999]
y = [-0.69999999999999996, -0.29999999999999999, -0.29999999999999999, -0.69999999999999996]
    def square_polygon_intersection(xmin, xmax, ymin, ymax, x, y):
        poly = Path(list(zip(x, y)))
        box = Bbox([[xmin, ymin], [xmax, ymax]])
        try:
>           clipped_poly = poly.clip_to_bbox(box)
E           AttributeError: 'Path' object has no attribute 'clip_to_bbox'
pvextractor/geometry/polygon.py:17: AttributeError

Alternatively, we could simply require a recent enough version of Matplotlib.

[keflavich]

@astrofrog - I picked the simpler solution. 1.1 is pretty old.