build: specify dependencies in a declarative way

as described in
https://www.gnu.org/software/emacs/manual/html_node/elisp/Simple-Packages.html
It fixes:
0xekez#3

htmlz-mode.el

@@ -1,10 +1,11 @@
-;;; htmlz-mode --- Simple real-time Emacs html preview
+;;; htmlz-mode.el --- Simple real-time Emacs html preview -*- lexical-binding:t -*-
 
 ;; Copyright (C) 2020 Zeke Medley
 
 ;; Author: Zeke Medley <[email protected]>
 ;; Keywords: html
-;; Version: 0.2
+;; Version: 0.3
+;; Package-Requires: ((websocket "1.14"))
 ;; URL: https://github.com/ZekeMedley/htmlz
 
 ;;; Commentary:
@@ -18,18 +19,9 @@
 ;;
 ;;; Code:
 
-(defun require-package (package)
-  "Install given PACKAGE if it was not installed before."
-  (if (package-installed-p package)
-      t
-    (progn
-      (unless (assoc package package-archive-contents)
-	(package-refresh-contents))
-      (package-install package))))
-
 (defun htmlz-init-dependencies ()
   "Initialize htmlz dependencies."
-  (require-package 'websocket)
+  (require 'websocket)
   (require 'package)
   (require 'browse-url)
   (require 'websocket))