Fix recognition of jquery to also work with non-globals

When jquery is not attached globally (e.g when using `browserify` or `requirejs`), the previous detection for jQuery would fail and think it would be Zepto, even if jQuery is used. Since `$` is the local variable for jQuery/Zepto, this commit makes the distinction between jQuery and Zepto work with a non-global jQuery.

`$.fn.jquery` is undefined in Zepto, but in jQuery it returns the version number and thus the expression returns `true`.