Fix platform update command (#2910)
Fix platform update command (#2910)

`tns platform update` command should install the latest platform in case it is not installed. Currently this fails (regression from previous commit).
Fix this by calling correct validate method.
NativeScript/nativescript-cli,NativeScript/nativescript-cli,NathanaelA/nativescript-cli,NativeScript/nativescript-cli,NathanaelA/nativescript-cli,NativeScript/nativescript-cli,NathanaelA/nativescript-cli,NathanaelA/nativescript-cli