MAINT: removed biom import

Fixes #524 

This _should_ fix the issue with Read The Docs not building the documentation. The other thing is specifying a requirements file but I think that will get nasty or weird because of numpy/scipy
The issue is that doing `import biom` requires that `biom` is installed, which was causing read the docs to bomb. You can do a virtualenv with readthedocs, but then you also need to have numpy and scipy installed, which I think is doable, but quickly gets nasty