Bump ajv from 6.12.0 to 6.12.6 in /src

Bumps [ajv](https://github.com/ajv-validator/ajv) from 6.12.0 to 6.12.6.
- [Release notes](https://github.com/ajv-validator/ajv/releases)
- [Commits](https://github.com/ajv-validator/ajv/compare/v6.12.0...v6.12.6)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: indirect
...

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

README.md

@@ -2,8 +2,7 @@
 Cucko is a Google Chrome browser extension that lets you hide the number of likes, retweets and replies a tweet has.
 
 ## Installation
-
-You can no longer install this extension.
+[Download the extension](https://chrome.google.com/webstore/detail/cuckoo-for-twitter/agjappnhmfoffegbhnakelgilanmnibb) from the Chrome web store here.
 
 ## Development
 To get set up for local development, follow these steps:

src/package-lock.json

@@ -148,9 +148,9 @@
       "dev": true
     },
     "ajv": {
-      "version": "6.12.0",
-      "resolved": "https://registry.npmjs.org/ajv/-/ajv-6.12.0.tgz",
-      "integrity": "sha512-D6gFiFA0RRLyUbvijN74DWAjXSFxWKaWP7mldxkVhyhAV3+SWA9HEJPHQ2c9soIeTFJqcSdFDGFgdqs1iUU2Hw==",
+      "version": "6.12.6",
+      "resolved": "https://registry.npmjs.org/ajv/-/ajv-6.12.6.tgz",
+      "integrity": "sha512-j3fVLgvTo527anyYyJOGTYJbG+vnnQYvE0m5mmkc1TK+nxAppkCLMIL0aZ4dblVCNoGShhm+kzE4ZUykBoMg4g==",
       "dev": true,
       "requires": {
         "fast-deep-equal": "^3.1.1",