Make deprecation messages consistent with those in 0.13.x

Author: farmdawgnation

core/src/main/scala/execution.scala

@@ -65,15 +65,15 @@ object Http extends Http(
     this.closeAndConfigure(builder => builder)
   }
 
-  @deprecated("Using the Http singleton directly will not be allowed in Dispatch 0.13.x. Please switch to invoking Http.default for using a globally accessible default Http client.", "0.12.2")
+  @deprecated("Using the Http singleton directly is deprecated and will be removed in a future version of dispatch. Please switch to invoking Http.default for using a globally accessible default Http client.", "0.12.2")
   override def apply(req: Req)
            (implicit executor: ExecutionContext): Future[Response] = super.apply(req)
 
-  @deprecated("Using the Http singleton directly will not be allowed in Dispatch 0.13.x. Please switch to invoking Http.default for using a globally accessible default Http client.", "0.12.2")
+  @deprecated("Using the Http singleton directly is deprecated and will be removed in a future version of dispatch. Please switch to invoking Http.default for using a globally accessible default Http client.", "0.12.2")
   override def apply[T](pair: (Request, AsyncHandler[T]))
               (implicit executor: ExecutionContext): Future[T] = super.apply(pair)
 
-  @deprecated("Using the Http singleton directly will not be allowed in Dispatch 0.13.x. Please switch to invoking Http.default for using a globally accessible default Http client.", "0.12.2")
+  @deprecated("Using the Http singleton directly is deprecated and will be removed in a future version of dispatch. Please switch to invoking Http.default for using a globally accessible default Http client.", "0.12.2")
   override def apply[T]
     (request: Request, handler: AsyncHandler[T])
     (implicit executor: ExecutionContext): Future[T] = super.apply(request, handler)