type of subprotocol is str, not List[str] (#308)

* type of subprotocol is str, not List[str]

* Force upgrade cached requirements

* Attempt to force clean test re-run

* Always force latest requirements

.travis.yml

@@ -13,7 +13,7 @@ services:
     - postgresql
 
 install:
-    - pip install -r requirements.txt
+    - pip install -U -r requirements.txt
 
 before_script:
     - psql -c 'create database starlette;' -U postgres

starlette/websockets.py

@@ -107,7 +107,7 @@ class WebSocket(HTTPConnection):
         else:
             raise RuntimeError('Cannot call "send" once a close message has been sent.')
 
-    async def accept(self, subprotocol: typing.List[str] = None) -> None:
+    async def accept(self, subprotocol: str = None) -> None:
         if self.client_state == WebSocketState.CONNECTING:
             # If we haven't yet seen the 'connect' message, then wait for it first.
             await self.receive()