two pandas compatibility changes for double indexing ix[..][..] were missed before
test_gmm and stats.anova
based on Warnings running with pandas master
BUG fix more ix indexing cases for pandas compat closes #1891
These are the last warnings or problems that I saw with pandas master
Coverage remained the same when pulling ec9d839 on josef-pkt:BUG_pandas_compat_ix into 300d997 on statsmodels:master.
all green, merging