Upgrade: Bump ajv from 6.7.0 to 6.8.1

Bumps [ajv](https://github.com/epoberezkin/ajv) from 6.7.0 to 6.8.1.
- [Release notes](https://github.com/epoberezkin/ajv/releases)
- [Commits](ajv-validator/ajv@v6.7.0...v6.8.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

packages/hint/package.json

@@ -8,7 +8,7 @@
   },
   "bin": "./dist/src/bin/hint.js",
   "dependencies": {
-    "ajv": "^6.7.0",
+    "ajv": "^6.8.1",
     "applicationinsights": "^1.1.0",
     "async": "^2.6.1",
     "boxen": "^2.1.0",

packages/parser-manifest/package.json

@@ -9,7 +9,7 @@
   "description": "webhint parser for web app manifest files",
   "devDependencies": {
     "@types/sinon": "^7.0.5",
-    "ajv": "^6.7.0",
+    "ajv": "^6.8.1",
     "ava": "^1.2.0",
     "cpx": "^1.5.0",
     "eslint": "^5.13.0",

yarn.lock

@@ -1189,10 +1189,10 @@ ajv-keywords@^3.1.0:
   resolved "https://registry.yarnpkg.com/ajv-keywords/-/ajv-keywords-3.3.0.tgz#cb6499da9b83177af8bc1732b2f0a1a1a3aacf8c"
   integrity sha512-CMzN9S62ZOO4sA/mJZIO4S++ZM7KFWzH3PPWkveLhy4OZ9i1/VatgwWMD46w/XbGCBy7Ye0gCk+Za6mmyfKK7g==
 
-ajv@^6.1.0, ajv@^6.5.3, ajv@^6.5.5, ajv@^6.6.1, ajv@^6.7.0:
-  version "6.7.0"
-  resolved "https://registry.yarnpkg.com/ajv/-/ajv-6.7.0.tgz#e3ce7bb372d6577bb1839f1dfdfcbf5ad2948d96"
-  integrity sha512-RZXPviBTtfmtka9n9sy1N5M5b82CbxWIR6HIis4s3WQTXDJamc/0gpCWNGz6EWdWp4DOfjzJfhz/AS9zVPjjWg==
+ajv@^6.1.0, ajv@^6.5.3, ajv@^6.5.5, ajv@^6.6.1, ajv@^6.8.1:
+  version "6.8.1"
+  resolved "https://registry.yarnpkg.com/ajv/-/ajv-6.8.1.tgz#0890b93742985ebf8973cd365c5b23920ce3cb20"
+  integrity sha512-eqxCp82P+JfqL683wwsL73XmFs1eG6qjw+RD3YHx+Jll1r0jNd4dh8QG9NYAeNGA/hnZjeEDgtTskgJULbxpWQ==
   dependencies:
     fast-deep-equal "^2.0.1"
     fast-json-stable-stringify "^2.0.0"