skip_known_failure using skip, not knowfailureif

Thomas and I looked into getting the known failures to show
up as 'K', instead of 'E', but it involved grabbing a custom
nose plugin from numpy and didn't seem worth the trouble. So
I'm putting skip_known_failure back to use skip and show up
as 'S'.

IPython/testing/decorators.py

@@ -319,7 +319,7 b' skipif_not_numpy = skipif(module_not_available(\'numpy\'),"This test requires nump'
 
 skipif_not_sympy = skipif(module_not_available('sympy'),"This test requires sympy")
 
-skip_known_failure = knownfailureif(True,'This test is known to fail')
+skip_known_failure = skip('This test is known to fail')
 
 # A null 'decorator', useful to make more readable code that needs to pick
 # between different decorators based on OS or other conditions