Fix iterating all GitLab comments

Fixes the following warning:

    reviewrot/gitlabstack.py:227: UserWarning: Calling a `list()` method
    without specifying `get_all=True` or `iterator=True` will return a
    maximum of 20 items.

reviewrot/gitlabstack.py

@@ -222,7 +222,7 @@ def get_last_comment(self, mr):
             last comment (LastComment): Returns namedtuple LastComment
             with data related to last comment
         """
-        for note in mr.notes.list():
+        for note in mr.notes.list(iterator=True):
             if not note.system:
                 return LastComment(
                     author=note.author["username"],