Onesignal Plugin Install Fails

I’ve created a brand new app on iOS.

Output from ionic info

ionic info

cli packages: (/usr/local/lib/node_modules)

    @ionic/cli-utils  : 1.19.1
    ionic (Ionic CLI) : 3.19.1

global packages:

    cordova (Cordova CLI) : 8.0.0 

local packages:

    @ionic/app-scripts : 3.1.8
    Cordova Platforms  : ios 4.5.4
    Ionic Framework    : ionic-angular 3.9.2


    ios-deploy : 1.8.6 
    ios-sim    : 5.0.13 
    Node       : v6.10.2
    npm        : 5.6.0 
    OS         : OS X El Capitan
    Xcode      : Xcode 7.3.1 Build version 7D1014 

Environment Variables:

    ANDROID_HOME : not set


    backend : pro

I create the new app, then run the following:

ionic cordova plugin add onesignal-cordova-plugin
> cordova plugin add onesignal-cordova-plugin --save
Installing "onesignal-cordova-plugin" for ios

Plugin doesn't support this project's apple-xcode version. apple-xcode: 7.3.1, failed version requirement: >=8.0.0

Skipping 'onesignal-cordova-plugin' for ios
Adding onesignal-cordova-plugin to package.json
Saved plugin info for "onesignal-cordova-plugin" to config.xml

I have been using XCode 7.3.1 and have also just upgraded to XCode 8.0.0 but still the same error.

Anyone have any idea how to get the plugin to install?
What am I missing?