Remove dependency on canvas (#354)

canvas/README.md

@@ -0,0 +1,7 @@
+# Why?
+
+This adds a mock module for `canvas`. Nteract has a ignored require and undeclared dependency on this module. `cavnas` is a server side node module and is not used in browser side code for nteract.
+
+Installing it locally (`npm install canvas`) will resolve the problem, but it is a native module so it is flaky depending on the system, node version, processor arch, etc. This module provides a simpler, more robust solution.
+
+Remove this workaround if [this bug](https://github.com/nteract/any-vega/issues/2) ever gets resolved 

canvas/index.js

@@ -0,0 +1 @@
+module.exports = {}

canvas/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "canvas",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "author": "",
+  "license": "ISC"
+}