fix: package.json & .snyk to reduce vulnerabilities (#39)

The following vulnerabilities are fixed with a Snyk patch:
- https://snyk.io/vuln/npm:debug:20170905

Latest report for dequelabs/axe-cli:
https://snyk.io/test/github/dequelabs/axe-cli

.snyk

@@ -0,0 +1,8 @@
+# Snyk (https://snyk.io) policy file, patches or ignores known vulnerabilities.
+version: v1.7.1
+ignore: {}
+# patches apply the minimum changes required to fix a vulnerability
+patch:
+  'npm:debug:20170905':
+    - phantomjs-prebuilt > extract-zip > debug:
+        patched: '2017-09-28T08:21:02.708Z'

package.json

@@ -6,7 +6,10 @@
   "engines": {
     "node": ">=6"
   },
-  "scripts": {},
+  "scripts": {
+    "snyk-protect": "snyk protect",
+    "prepublish": "npm run snyk-protect"
+  },
   "repository": {
     "type": "git",
     "url": "git+https://github.com/dequelabs/axe-cli.git"
@@ -38,6 +41,8 @@
     "colors": "^1.1.2",
     "commander": "^2.9.0",
     "phantomjs-prebuilt": "^2.1.13",
-    "selenium-webdriver": "^3.0.0"
-  }
+    "selenium-webdriver": "^3.0.0",
+    "snyk": "^1.41.1"
+  },
+  "snyk": true
 }