DOC/BLD: Need yaml for logging, but not yaml_serialize. Also, specify dep versions in README.

In the wake of the Travis mess, I think the default conda install will not work because of #78 . These new installation instructions give a ``conda install ...`` command with version numbers that work, same as those tested on Travis.

If #78 is fixed, default conda should work again, and we can revert to the simple instructions if we want. But it might be better to stay specific -- this could happen again on the next conda update.