Remove skbio deps from readme

@gregcaporaso and I chatted about this (in the context of scikit-bio and [IAB](http://applied-bioinformatics.org)) and thought it'd be a good idea to remove the explicit dependency list from the README in both projects. This list is nearly always outdated and `pip` should handle installing the dependencies. If someone wants to see the exact dependencies required by skbio, they can always look in setup.py.