src: disable harmony classes

The V8 development branch has unshipped ES6 classes pending resolution
of a number of inheritance edge cases.  Disable classes in io.js for
the sake of feature parity.

See https://github.com/iojs/io.js/issues/251 for background and
discussion.

PR-URL: https://github.com/iojs/io.js/pull/272


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Domenic Denicola <domenic@domenicdenicola.com>