Update metropolis.py

Small edit to make DEMetropolis visible.

Author: junpenglao

pymc3/step_methods/metropolis.py

@@ -9,7 +9,7 @@
 import pymc3 as pm
 from pymc3.theanof import floatX
 
-__all__ = ['Metropolis', 'BinaryMetropolis', 'BinaryGibbsMetropolis',
+__all__ = ['Metropolis', 'DEMetropolis', 'BinaryMetropolis', 'BinaryGibbsMetropolis',
            'CategoricalGibbsMetropolis', 'NormalProposal', 'CauchyProposal',
            'LaplaceProposal', 'PoissonProposal', 'MultivariateNormalProposal']