Update grunt-contrib-htmlmin dependecy -> fix custom element errors

In the process of minifying html, in the `grunt build` process, the generated html breaks on custom elements with dashes in their names, like `<logo-anim>`. This was a problem with the html-minifier, that is already fixed, but the dependency needs to be updated.

Original (html-minifier) issue here -> https://github.com/kangax/html-minifier/issues/36