add license to package.json (#178)

Because right now it doesn't show any license on
[the npm page](https://www.npmjs.com/package/prettier).

Also, it's causing my [DependencyCI](https://dependencyci.com/github/kentcdodds/prettier-eslint/builds/5)
build to fail 😅

package.json

@@ -9,6 +9,7 @@
     "url": "git+https://github.com/jlongster/prettier.git"
   },
   "author": "James Long",
+  "license": "MIT",
   "bugs": {
     "url": "https://github.com/jlongster/prettier/issues"
   },