Update dependency botkit to v0.6.18

Author: renovate[bot]

language/slackbot/package.json

@@ -24,7 +24,7 @@
   },
   "dependencies": {
     "@google-cloud/language": "0.11.0",
-    "botkit": "0.5.7",
+    "botkit": "0.6.18",
     "sqlite3": "3.1.9"
   },
   "devDependencies": {