made travis build_sphinx command not do intersphinx

By default, the sphinx build uses intersphinx, which links documentation to other sphinx docs like scipy or numpy. This is good for the actual docs, but causes a sphinx build warning if any of the other sites are offline or the web momentarily hiccups.  In #760 this led to a false sphinx build warning on travis.  So this just turns off intersphinx for the travis doc build to prevent that in the future.