Removed unneeded Matplot import

This import resulted in unnecessary failure of the test suite if
matplotlib is not installed.
Merge pull request #501 from tobeplugged/fix_plot_tests

Removed unneeded Matplot import
categorical_like now casts values to integer, rather than failing
Merge branch '2.3' of github.com:pymc-devs/pymc into 2.3
Changed test to accomodate categorical int casting