CI: xlrd PendingDeprecationWarning
https://travis-ci.org/pandas-dev/pandas/jobs/427910477

```
E           AssertionError: Caused unexpected warning(s): [('PendingDeprecationWarning', PendingDeprecationWarning("This method will be removed in future versions.  Use 'tree.iter()' or 'list(tree.iter())' instead.",), '/home/travis/miniconda3/envs/pandas/lib/python2.7/site-packages/xlrd/xlsx.py', 246), ('PendingDeprecationWarning', PendingDeprecationWarning("This method will be removed in future versions.  Use 'tree.iter()' or 'list(tree.iter())' instead.",), '/home/travis/miniconda3/envs/pandas/lib/python2.7/site-packages/xlrd/xlsx.py', 292), ('PendingDeprecationWarning', PendingDeprecationWarning("This method will be removed in future versions.  Use 'tree.iter()' or 'list(tree.iter())' instead.",), '/home/travis/miniconda3/envs/pandas/lib/python2.7/site-packages/xlrd/xlsx.py', 246)].
```