Attempt to fix TravisCI install failures for Matplotlib by using conda
Tweak to .travis.yml to eliminate unneeded packages
Added conda install logic to .travis.yml
Fix for DISPLAY runtime error in Travis
Potential fix for MPL display issues
Another desperate attempt to exorcise MPL errors
Removed all calls to show() in examples
Almost ready to give up on MPL
Fixing MPL bug
More MPL debugging
Experimenting with import order
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.
Experimenting with import order
Merge branch 'fix-mpl-travis-function-imports' of git://github.com/kyleam/pymc into fix_travis
Attempt to pass tests using conda to install
Merge pull request #475 from pymc-devs/fix_travis

Fix for Matplotlib issues in TravisCI
pip install numdifftools after other conda packages to avoid pip installing of numpy and scipy.