Depend on slackhq/hubot-slack for robot.react

Added via slackapi/hubot-slack#363. When a new release beyond 4.10.0 is
released with this change, a future commit will update the dependency
accordingly.

package.json

@@ -11,8 +11,8 @@
   },
   "dependencies": {},
   "peerDependencies": {
-    "hubot": "2.x",
-    "hubot-slack": "4.x"
+    "hubot": "^2.19.0",
+    "hubot-slack": "slackhq/hubot-slack"
   },
   "devDependencies": {
     "browserify": "^13.0.0",
@@ -28,7 +28,7 @@
     "gulp-istanbul": "^0.10.3",
     "gulp-mocha": "^2.2.0",
     "hubot": "^2.19.0",
-    "hubot-slack": "^4.10.0",
+    "hubot-slack": "slackhq/hubot-slack",
     "hubot-test-helper": "^1.3.0",
     "istanbul": "^0.4.1",
     "mocha": "^2.3.4",