The problem is that the 1.1.0 version of the plugin was released today with that requirement, but cordova-ios 4 is not released yet (is still in development). So if you wanna keep using cordova-ios 3.8 while the stable version 4 is released, you have to install the previous version of the plugin (1.0.0).
cordova plugin add cordova-plugin-whitelist@1.0.0
I'm not sure if you can run this command using Ionic. Maybe you have to edit the ios.json with the version specified?
If you can't do this, maybe you have to wait until cordova-ios 4 is released
I created an issue https://issues.apache.org/jira/browse/CB-9232
Edit:
From the issue created:
"If you have cordova-ios 3.8.0 installed, it won't install the plugin for the ios platform (but will do so for the others if they fulfill the engine requirement). The plugin only works for cordova-ios 4.0.0 which has a different whitelist mechanism based on CSP. cordova-ios 3.8.0 still uses the old tag."
So, for now (while the new version of the tools of cordova-ios 4 is released, you can ignore this error)
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