uncomment test (#2369)

Author: tv3141

.travis.yml

@@ -95,9 +95,7 @@ install:
   - python xarray/util/print_versions.py
 
 script:
-  # TODO: restore this check once the upstream pandas issue is fixed:
-  # https://github.com/pandas-dev/pandas/issues/21071
-  # - python -OO -c "import xarray"
+  - python -OO -c "import xarray"
   - if [[ "$CONDA_ENV" == "docs" ]]; then
       conda install -c conda-forge sphinx sphinx_rtd_theme sphinx-gallery numpydoc;
       sphinx-build -n -j auto -b html -d _build/doctrees doc _build/html;