src: use Number::New() for heapTotal/heapUsed

With --max_old_space_size=12345 it's possible to create a JS heap that
is larger than what fits in an unsigned int so use Number::New() rather
than Integer::NewFromUnsigned().

Performance-wise, it doesn't matter much.  If V8 can fit the double in
a SMI, it will.

PR-URL: https://github.com/iojs/io.js/pull/1148


Reviewed-By: Trevor Norris <trev.norris@gmail.com>