Bump ajv from 5.5.2 to 6.12.3

Bumps [ajv](https://github.com/ajv-validator/ajv) from 5.5.2 to 6.12.3.
- [Release notes](https://github.com/ajv-validator/ajv/releases)
- [Commits](ajv-validator/ajv@v5.5.2...v6.12.3)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

npm-shrinkwrap.json

@@ -4,7 +4,7 @@
   "description": "Based on PatternLab and Foundation - [Pattern Lab Website](http://patternlab.io/) - [About Pattern Lab](http://patternlab.io/about.html) - [Documentation](http://patternlab.io/docs/index.html) - [Demo](http://demo.patternlab.io/)",
   "main": "source/js/inc/app.js",
   "devDependencies": {
-    "ajv": "^5.2.2",
+    "ajv": "^6.12.3",
     "autoprefixer": "^6.0.3",
     "exports-loader": "0.6.3",
     "grunt": "~0.4.5",