Address a test failure w/ scipy.stats.distributions named arguments

A recent PR in scipy.stats allowing using named arguments for distributions (https://github.com/scipy/scipy/pull/2588 and https://github.com/rgommers/scipy/tree/pr/2588) brings in a failure in `statsmodels.sandbox.distributions.tests.testtransf.Test_Transf2.test_equivalent`. 

This PR fixes the failure, at least superficially: the test passes, but it really needs to be reviewed by somebody who's familiar with the `TransfTwo_gen` code. The fix here assumes that the attribute `shape` of the `TransfTwo_gen` class is intended to correspond to the `shapes` attribute of the `stats.rv_continuous` (note the singular/plural shape/shapes).