update some qiime/qiime urls to biocore/qiime

The repo has moved, and the GitHub API (used by nbviewer) gives 404 on the removed qiime/qiime repo.
Merge pull request #1536 from minrk/biocore-urls

update some qiime/qiime urls to biocore/qiime
skbio.maths -> skbio.math
Merge pull request #1537 from ElBrogrammer/brah-i-learned-maths

skbio.maths -> skbio.math