Require pandas >= 0.18.0 (#34)

According to pandas documentation: `RangeIndex is a sub-class of Int64Index added in version 0.18.0`
RangeIndex is used in root_pandas.

Author: sashabaranov

setup.py

@@ -9,7 +9,7 @@
       license='MIT',
       install_requires=[
           'numpy',
-          'pandas',
+          'pandas>=0.18.0',
           'root_numpy',
       ],
       packages=['root_pandas'],