Revert "Experimenting with import order"

This reverts commit c407a00, which selected the Agg backend for
Matplotlib in pymc/__init__.py, overriding the effects of 40a8070. These
changes were unnecessary to fix the non-interative display errors in the
Travis tests and prevent interactive plotting unless the user has
selected a backend prior to importing pymc.
Merge pull request #478 from kyleam/remove-mpl-agg-init

Revert "Experimenting with import order"