Revert "[#57] Don't install the most recent version of openpyxl"

This reverts commit 25b2003.

Author: Bjwebb

setup.py

@@ -1,7 +1,7 @@
 from setuptools import setup
 import sys
 
-install_requires = ['jsonref', 'schema', 'openpyxl>=2,<2.2', 'six', 'pytz']
+install_requires = ['jsonref', 'schema', 'openpyxl>=2', 'six', 'pytz']
 
 if sys.version < '3':
     install_requires.append('unicodecsv')