Bump react to 15.6 to fix build problems

matrix-react-test-tools requires react 15.6 or later, so make sure we require
it in riot-web too.

package.json

@@ -70,10 +70,10 @@
     "matrix-react-sdk": "0.9.7",
     "modernizr": "^3.1.0",
     "pako": "^1.0.5",
-    "react": "^15.4.0",
+    "react": "^15.6.0",
     "react-dnd": "^2.1.4",
     "react-dnd-html5-backend": "^2.1.2",
-    "react-dom": "^15.4.0",
+    "react-dom": "^15.6.0",
     "react-gemini-scrollbar": "matrix-org/react-gemini-scrollbar#5e97aef",
     "sanitize-html": "^1.11.1",
     "text-encoding-utf-8": "^1.0.1",
@@ -135,7 +135,7 @@
     "postcss-simple-vars": "^3.0.0",
     "postcss-strip-inline-comments": "^0.1.5",
     "react-addons-perf": "^15.4.0",
-    "react-addons-test-utils": "^15.4.0",
+    "react-addons-test-utils": "^15.6.0",
     "rimraf": "^2.4.3",
     "source-map-loader": "^0.1.5",
     "webpack": "^1.12.14",