Add a files array to `package.json`

Ensures that only and all of the `lib/` directory will be in the release.

package.json

@@ -3,6 +3,10 @@
   "version": "1.1.0",
   "description": "Another JSON Schema Validator",
   "main": "lib/ajv.js",
+  "files": [
+    "lib/",
+    "LICENSE"
+  ],
   "scripts": {
     "test-spec": "mocha spec/*.spec.js -R spec",
     "test-cov": "istanbul cover -x '**/spec/**' node_modules/mocha/bin/_mocha -- spec/*.spec.js -R spec",