Work around the regression introduced in matplotlib 1.3.1 by having `Quantity.ravel` return a dimensionless array. Emit a warning explaining the issue. Closes #245
Apply "unitless ravel()" workaround only for matplotlib 1.3.1
Make tests aware of the "unitless ravel()" workaround
Merge pull request #274 from brian-team/workaround_matplotlib_bug

Work around the regression introduced in matplotlib 1.3.1