YapfBear: Update yapf requirement to 0.11

This was already changed in requirements.txt, but was forgotten in the
bear class.

Closes coala#742

bears/python/YapfBear.py

@@ -19,7 +19,7 @@ class YapfBear(LocalBear):
     """
     LANGUAGES = {"Python", "Python 2", "Python 3"}
     AUTHORS = {'The coala developers'}
-    REQUIREMENTS = {PipRequirement('yapf', '0.10')}
+    REQUIREMENTS = {PipRequirement('yapf', '0.11')}
     AUTHORS_EMAILS = {'[email protected]'}
     LICENSE = 'AGPL-3.0'
     CAN_FIX = {'Formatting'}