Change readyState needed to "interactive"

The document readyState is "interactive" after DOMContentLoaded,
and "complete" after load.

If montage.js loaded after DOMContentLoaded/"interactive" but
before load/"complete", then domLoad would never be called. This
commit fixes this.
Merge pull request #1089 from Stuk/readystate

Change an occurrence of readyState to "interactive" so we can use the "async"
attribute on the Montage bootstrapping script.