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.