{"2":"aaron@mail.com","1":"bernard@mail.com"}

$.getJSON('urlwithjsonresponse.com',function(response){
    console.log(response);
});

{1:"bernard@mail.com",2:"aaron@mail.com"}

$a = [ 2 => "aaron@mail.com", 1 => "bernard@mail.com" ];
$a = array_values( $a );
exit( json_encode( $a ) );

$a = [
    2 => [ "id" => 2, "email" => "aaron@mail.com" ],
    1 => [ "id" => 1, "email" => "bernard@mail.com" ]
];
exit( json_encode( array_values( $a ) );