Add crypto into the required applications

As suggested by @prof3ta.

src/cowboy.app.src

@@ -20,7 +20,8 @@
 	{applications, [
 		kernel,
 		stdlib,
-		ranch
+		ranch,
+		crypto
 	]},
 	{mod, {cowboy_app, []}},
 	{env, []}

src/cowboy_websocket.erl

@@ -13,10 +13,6 @@
 %% OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
 %% @doc WebSocket protocol implementation.
-%%
-%% When using websockets, make sure that the crypto application is
-%% included in your release. If you are not using releases then there
-%% is no need for concern as crypto is already included.
 -module(cowboy_websocket).
 
 %% API.