There is an issue with the recent fastai conda packages if you have an outdated anaconda package.
This is caused by an outdated anaconda package, which wants a numpy < 1.15
do
conda install anaconda
which should install anaconda 5.3.0 or higher.
and then it should work.
I changed meta.yaml deps to not ask for numpy>=1.15, but rolled back to >=1.12 so the next release should take care of it.