Use Buffer.from instead of new Buffer (set minimum node version to 6.0.0)

See:
 - https://nodejs.org/api/deprecations.html#deprecations_dep0005_buffer_constructor
 - https://nodejs.org/api/buffer.html#buffer_class_method_buffer_from_string_encoding

Author: ForbesLindesay

package.json

@@ -39,6 +39,9 @@
     "pretest": "npm run build",
     "test": "node test/index && node test/cache && node test/cache-invalidation && rimraf lib/cache"
   },
+  "engines": {
+    "node": ">=6.0.0"
+  },
   "repository": {
     "type": "git",
     "url": "https://github.com/ForbesLindesay/http-basic.git"

src/index.ts

@@ -72,7 +72,7 @@ function _request(method: HttpVerb, url: string, options: Options, callback: Cal
   var rawHeaders = options.headers || {};
   var headers = caseless(rawHeaders);
   if (urlObject.auth) {
-    headers.set('Authorization', 'Basic ' + (new Buffer(urlObject.auth)).toString('base64'));
+    headers.set('Authorization', 'Basic ' + (Buffer.from(urlObject.auth)).toString('base64'));
   }
   var agent = 'agent' in options ? options.agent : false;