Fix traceback postmortem to not print one character per line.

Fixes https://github.com/Yelp/Testify/issues/188

I'm not really sure how to test this but here it is working:

{{{
$ python failing_test.py -d

DEBUGGER
Traceback (most recent call last):
  File "failing_test.py", line 6, in test_fail
    assert False
AssertionError

> /home/anthony/workspace/Testify/failing_test.py(6)test_fail()
      5     def test_fail(self):
----> 6         assert False
      7 

ipdb> q
fail: TestFailingTest.test_fail
Traceback (most recent call last):
  File "failing_test.py", line 6, in test_fail
    assert False
AssertionError

F
FAILED.  1 test / 1 case: 0 passed, 1 failed.  (Total test time 0.00s)

}}}