update debug to 2.6.9

Resolves audit warning for https://www.npmjs.com/advisories/534

This issue does not directly affect localtunnel since it does not use
the `o` formatter.

package.json

@@ -10,7 +10,7 @@
   },
   "dependencies": {
     "axios": "0.17.1",
-    "debug": "2.6.8",
+    "debug": "2.6.9",
     "openurl": "1.1.1",
     "yargs": "6.6.0"
   },
@@ -24,4 +24,4 @@
     "lt": "./bin/client"
   },
   "main": "./client.js"
-}
+}