Do not treat translate attribtues as boolean values

See #172 and the current draft of the HTML5 spec:
http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#the-translate-attribute

Note that the `translate` spec is somewhat confusing, as elements inherit their `translate` value from their parent node(s), which is a fairly unusual behavior.  Thus, `translate` appears to be exposed to the DOM as a boolean, but requires a string value of `"yes"` or `"no"` to be set in the HTML document itself (ergo, it is not a "boolean" attribute from html-minifier's perspective).

Given that no browsers currently implement this API, and a few [libraries](http://angular-translate.github.io) happen to use custom `translate` attributes in the DOM, it seems pretty safe to remove this for now.