Add Optional[] for all remaining cases of x: <type> = None (#1424)

* Final round of adding Optional[] to type of arguments with default = None
* Update Travis to use --no-implicit-optionals and clarify CONTRIBUTING.md

stdlib/3.4/asyncio/locks.pyi

@@ -37,7 +37,7 @@ class Event:
     def wait(self) -> Generator[Any, None, bool]: ...
 
 class Condition(_ContextManagerMixin):
-    def __init__(self, lock: Lock = None, *, loop: Optional[AbstractEventLoop] = ...) -> None: ...
+    def __init__(self, lock: Optional[Lock] = None, *, loop: Optional[AbstractEventLoop] = ...) -> None: ...
     def locked(self) -> bool: ...
     @coroutine
     def acquire(self) -> Generator[Any, None, bool]: ...