Upgrade yarn and set node version correctly

Author: vongrippen

Diff for: .tool-versions

@@ -1 +1 @@
-nodejs 6.3.1
+nodejs 6.12.3

Diff for: package.json

@@ -22,7 +22,7 @@
     "hubot-rules": "^0.1.1",
     "hubot-scripts": "^2.16.2",
     "hubot-shipit": "^0.2.0",
-    "hubot-slack": "^3.4.2",
+    "hubot-slack": "4.5.1",
     "hubot-slack-blacklist-middleware": "^1.0.2",
     "hubot-slack-whitelist-middleware": "^1.0.1",
     "hubot-youtube": "^1.0.2",