[#457] Only display phonegap.js injection when used.

When a user does not use phonegap.js, then no message or injection
will occur.

When a user referenced phonegap.js, then following is shown:

    $ phonegap build android
     [warning] phonegap.js support will soon be removed.
     [warning] please replace 'phonegap.js' references with 'cordova.js'
    [phonegap] adding phonegap.js to the android platform
    [phonegap] executing 'cordova build android'...
    [phonegap] successfully executed 'cordova build android'