Add boolean? generator

In clojure I can do
`(gen/generate (s/gen int?)) ;; => 1094`
or
`(gen/generate (s/gen boolean?)) ;; => false`.

In clojurescript I can do
`(gen/generate (s/gen int?)) ;; => -308`.
But in clojurescript I can't do
`(gen/generate (s/gen boolean?)) ;; => Error: Unable to construct gen at: [] for: function cljs$core$boolean_QMARK_{return (x === true) || (x === false);}].`

As far as I can tell it's because there is no boolean generator.