TST: allow to check for specific xlrd version and skip reader test if xlrd < 0.9
See http://nipy.bic.berkeley.edu/builders/pandas-py2.x-wheezy-sparc/builds/148/steps/shell_4/logs/stdio for failure when outdated xlrd found (e.g. on debian stable wheezy)

@cpcloud @jtratner seems reasaonable....
 fine with me. At some point probably better to use LooseVersion, but seems like a good fix.
 minimum version I can get to pass the test suite is 0.8.
