BUG: Fixes for 2.6 test failures, replacing astype(str) with apply(str)

Simple fix for 2 test failures on 2.6.  See

https://travis-ci.org/bashtage/statsmodels/builds/21917286

build 209.1.  Build 209 uses the travis-miniconda to test against 2.6, but this branch uses the older build config.