Make constrained MOO tutorial backward compatible with old matplotlib (#1970)

Summary:
The recent change to use `matplotlib.colormaps` instead of `plt.cm.get_cmap` broke the tutorial on older versions of matplotlib. This reverts this change.

Pull Request resolved: #1970

Reviewed By: saitcakmak

Differential Revision: D48056932

Pulled By: Balandat

fbshipit-source-id: b17069624ea0c24ae947f876e8f985f3997a7045

Author: Balandat