[plugins.json] Add cocoapods-triumph-sdk-plugin

[posplaw]

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

{
  "gem": "cocoapods-triumph-sdk-plugin",
  "name": "CocoaPods Triumph SDK Plugin",
  "author": "Alex Oakley",
  "url": "https://github.com/triumpharcade/cocoapods-triumph-sdk-plugin",
  "description": "Helper plugin for installation of TriumphSDK as CocoaPod"
}