Updated Downloads Description

Right now the title for the second download on the Marionette page says `AMD/RequireJS`. Since RequireJS uses AMD, it may be considered superfluous to include that in the title as well. Either way, a slightly bigger concern is that download also seems to support CommonJS-like environments, like Node, but there's no information about that on the page.

This may have led to issues like [this one](https://github.com/marionettejs/backbone.marionette/issues/880) where a user was unable to determine how to load Marionette in a CommonJS-like environment (I *think* Components is CommonJS-like, as Node is, but do correct me if you know otherwise).

This PR replaces 'RequireJS' with 'CommonJS' in the title, but there are some nuances to consider. Firstly, according to [this UMD pattern](https://github.com/umdjs/umd/blob/master/returnExports.js) it doesn't seem to support stricter CommonJS environments. Calling it `CommonJS` may confuse users in those stricter environments (though I admit I don't know what those environments may be).

Something else to consider is that the second download also works as a browser global. I'm sure it was intentionally split up, but maybe it'd be easier to just have a single download? That is how Backbone works, so I don't think it'd be too confusing to most users. The cost is a slightly larger file, but the size might be considered negligible compared to the size of the rest of jquery, underscore, backbone, and marionette.