Merge pull request #1945 from mozilla/greenkeeper/concurrently-3.4.0

Update concurrently to the latest version 🚀

package.json

@@ -239,7 +239,7 @@
     "chai": "3.5.0",
     "chalk": "1.1.3",
     "cheerio": "0.22.0",
-    "concurrently": "3.3.0",
+    "concurrently": "3.4.0",
     "cookie": "0.3.1",
     "css-loader": "0.27.3",
     "deepcopy": "0.6.3",