Stub defines are wrong for jspm bundle-sfx #12

Re. https://github.com/Hypercubed/systemjs-plugin-html/blob/master/html-builder.js#L75

% jspm --version
0.16.48
Running against local jspm install.

While building an sfx bundle, it's expected that SystemJS would not be required, but building a bundle with systemjs-pugin-html enabled breaks the promise:

% jspm bundle-sfx src/index.ts

Generates:

...
$__System.register("1", ["2", "22"], function (exports_1, context_1) {
    ...
});

System.register('bower:polymer-ts@0.1.28/polymer-ts.html!github:Hypercubed/systemjs-plugin-html@0.0.8.js', [], false, function() {});
...

I verified that changing System.register to $__System.register works as expected. jspm bundle also works as expected.