Use slash in user agent instead of space

Now "Flamenco-Worker/{version}" is used, instead of
"Flamenco-Worker {version}", to comply with RFC 7231 section 5.5.3
https://tools.ietf.org/html/rfc7231#section-5.5.3

flamenco_worker/upstream.py

@@ -38,7 +38,7 @@ class FlamencoManager:
 
     @functools.lru_cache(1)
     def user_agent(self):
-        return 'Flamenco-Worker %s' % self.flamenco_worker_version
+        return 'Flamenco-Worker/%s' % self.flamenco_worker_version
 
     async def client_request(self, method, url, *,
                              params=None,