BLD: Switch back to PyPI version of xlrd

The bug which prompted us to use our own version of xlrd is patched in
the current PyPI release, so switch back to it.

etc/requirements_dev.txt

@@ -2,7 +2,7 @@
 nose==1.3.7
 nose-parameterized==0.5.0
 nose-ignore-docstring==0.2
--e git://github.com/python-excel/xlrd.git@b9239bf2bfc42e088dd2a75c22164757cca9cc4f#egg=xlrd-master
+xlrd==0.9.4
 
 # These are required by mock or its dependencies
 MarkupSafe==0.23