Workaround for layout variable leak bug in Assemble.

This is a [workaround](http://stackoverflow.com/questions/24495789/assemble-io-yaml-frontmatter-variables-in-layout) for the [bug](https://github.com/assemble/assemble/issues/423) in Assemble that leaks layout variables to the global scope. Documentation on this added to the [Layouts](https://github.com/optimizely/marketing-website/wiki/Assemble-layouts) wiki page.
@kylerush this is something I keep seeing locally, too. This page doesn't use the benefits layout. So it's the same bug except this is happening for the `layout_body_class` variable invoked [here](https://github.com/optimizely/marketing-website/blob/master/website-guts/templates/layouts/wrapper.hbs#L29).

Do we want to set this layout variable to false on page.hbs?