BLD: Now compatible with Numba 0.12

Addresses #78. Now tests OK on old anaconda (numba 0.11) and new (numba 0.12).

There were major changes to the numba compiler, so that in-place subtraction (-=) is currently not supported. But in-place addition still is. Probably a temporary condition.

As predicted, silencing debug messages is no longer necessary, and in fact raises an error.