Review all lectures using `numba=0.49`

[mmcky]

We are getting reports of issues for numba=0.49 coming through on the QuantEcon package as well as lecture failures. There are some big changes in numba=0.49 due to restructuring the package. While it isn't in anaconda release yet -- if anyone updates numba (or packages that pull in the latest numba like interpolation) it becomes an issue.

We need to review all lectures in a controlled environment.

I suggest you use environment.yml - @najuzilu might be able to help you get up and running with `environments.

Please report back any issues here.