Fix openpyxl at pandas compatible version

requirements.txt

@@ -8,7 +8,7 @@ ipython==2.1.0
 matplotlib==1.3.1
 nose==1.3.3
 numpy==1.8.1
-openpyxl==2.0.4
+openpyxl==1.8.6
 pandas==0.14.0
 pyzmq==14.3.1
 scikit-learn==0.14.1