TST: Add xlrd module to requirements_dev

THe xlrd module is used by the answer_key module to read results
from a spreadsheet.

etc/requirements_dev.txt

@@ -2,6 +2,7 @@
 nose==1.3.0
 nose-parameterized==0.3
 nose-ignore-docstring==0.1
+xlrd=0.9.2
 
 # Linting