XFail old GMMHMM tests
XFail old GMMHMM tests

Don't pass since 934f26, and use the scikit-learn GMM class which is
deprecated. Mark them as xfail for now. Mark test_fit_zero_variance for full
covariance as xfail, since it has never worked (since 0.2.0 at least).

vchu/hmmlearn,hmmlearn/hmmlearn,vchu/hmmlearn,hmmlearn/hmmlearn