Fix Python 3.7 support, and run Travis tests on 3.7.

Travis' "nightly" python is actually older than 3.7.0 (3.7.0a4+)

.travis.yml

@@ -8,7 +8,7 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
-  - "nightly"
+  - "3.7-dev"
   - "pypy"
   - "pypy-5.3.1" # default pypy3 doesn't work, see https://bitbucket.org/pypy/pypy/issues/2129/local-variable-val-referenced-before
 env:

plugins/__init__.py

@@ -196,7 +196,6 @@ class ChannelUserDictionary(collections.MutableMapping):
         for channel, ids in self.channels.items():
             for id_, value in ids.items():
                 yield (channel, id_)
-        raise StopIteration()
 
     def __len__(self):
         return sum([len(x) for x in self.channels])