Point webapp at redux webapp-master branch (#6415)

webapp/package.json

@@ -22,7 +22,7 @@
     "localforage": "1.5.0",
     "marked": "mattermost/marked#8f5902fff9bad793cd6c66e0c44002c9e79e1317",
     "match-at": "0.1.0",
-    "mattermost-redux": "mattermost/mattermost-redux#webapp-3.9",
+    "mattermost-redux": "mattermost/mattermost-redux#webapp-master",
     "object-assign": "4.1.1",
     "pdfjs-dist": "1.7.363",
     "perfect-scrollbar": "0.6.16",