Travis says this is broken in 2.6 :( 

    ======================================================================  
    ERROR: test_attempt_read_encrypted_no_password_xls (test.test_read.ReadXlsTest)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/home/travis/build/okfn/messytables/test/test_read.py", line 308, in test_attempt_read_encrypted_no_password_xls
        with assert_raises(ReadError) as cm:
    TypeError: failUnlessRaises() takes at least 3 arguments (2 given)