html5lib update

[mgilson]

It looks like an update to html5lib is going to break this project once it is released. At least, I tried the latest html5lib and bleach wouldn't import:

>>> sys.path.insert(0, PATH_TO_NEW_HTML5LIB)
>>> import bleach
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Python/2.7/site-packages/bleach/__init__.py", line 13, in <module>
    from .sanitizer import BleachSanitizer
  File "/Library/Python/2.7/site-packages/bleach/sanitizer.py", line 11, in <module>
    PROTOS.remove('feed')
AttributeError: 'frozenset' object has no attribute 'remove'

I don't think there is any problem yet as html5lib hasn't actually released the latest version (as far as I'm aware), but I figured that I'd let you know so you can get ahead of this one...

[willkg]

This is covered by #195. Thank you for the heads-up!