Remove Modernizr module's dependency on html5shiv

The browser landscape was very different when I created this version of the Drupal module, and it is not as imperative to include html5shiv anymore. It's currently a hard-coded dependency in the Modernizr module itself. This should be turned into a configuration option, with the default being that it is NOT included.

Impact on current module users

None. People who rebuild Modernizr after upgrading the module will not lose this particular piece of their Modernizr build; the system will see that the old custom build had it, and include it in the list when they rebuild. However, for new sites (or people who manually de-select it on Modernizr.com) the module will no longer complain about the html5shiv being missing.