Fix conversion of boolean atom to string

mustache:render("{{banan}}", dict:from_list([{banan, true}])).

** exception error: no function clause matching mustache:escape(true,[])
     in function  erl_eval:do_apply/5
     in call from erl_eval:expr/5
     in call from erl_eval:expr/5
     in call from mustache:render/3

src/mustache.erl

@@ -190,8 +190,6 @@ to_s(Val) when is_integer(Val) ->
   integer_to_list(Val);
 to_s(Val) when is_float(Val) ->
   io_lib:format("~.2f", [Val]);
-to_s(Val) when is_boolean(Val) ->
-  Val;
 to_s(Val) when is_atom(Val) ->
   atom_to_list(Val);
 to_s(Val) ->