Switch back to mocha from ava. (#143)

* Switch back to mocha from ava.

* Make lint happy.

* Uncache vision.

* Make canvas dependency optional.

* Change node version.

* Remove const

Author: jmdobry

packages/google-cloud-vision/samples/package.json

@@ -9,9 +9,11 @@
   },
   "dependencies": {
     "async": "^1.5.0",
-    "canvas": "^1.3.15",
     "gcloud": "^0.32.0",
     "natural": "^0.4.0",
     "redis": "^2.6.0-2"
+  },
+  "optionalDependencies": {
+    "canvas": "^1.3.15"
   }
 }