Fix Travis Matplotlib errors with function imports

This commit makes two changes to avoid Matplotlib display errors from
non-interactive shells.

1. Use Agg backend in test_plots.py.
2. Import pyplot within plotting functions. This allows plotting
   functions to still be imported into top-level __init__.py without
   selecting a another backend before Agg can be selected in tests.

This also has the advantage that users running scripts in a
non-interactive shell will be able to specify the backend after the
top-level pymc import.
Merge branch 'fix-mpl-travis-function-imports' of git://github.com/kyleam/pymc into fix_travis
Attempt to pass tests using conda to install