Xfail test_sign_in_is_this_your_computer for Persona issue 4125

https://github.com/mozilla/persona/issues/4125
"is this your computer" screen not being shown for train-2014.04.16
Merge pull request #79 from bobsilverberg/fail

Xfail test_sign_in_is_this_your_computer for Persona issue 4125