[plugins.json] Add cocoapods-repo-update

[James Treanor]

Please add the following entry to the plugins.json file:

{
  "gem": "cocoapods-repo-update",
  "name": "CocoaPods Repo Update",
  "author": "James Treanor",
  "url": "https://github.com/wordpress-mobile/cocoapods-repo-update",
  "description": "cocoapods-repo-update is a CocoaPods plugin that checks your dependencies when you run `pod install` and updates the local specs repositories if needed."
}

[James Treanor]

PR here: #92

[James Treanor]

The PR is now merged.