bump node package version 1.6.0

Added Node.js `engine >= 8.0.0` restriction to use V8 version 6.X

Author: pmed

package.json

@@ -1,8 +1,9 @@
 {
   "name": "v8pp",
-  "version": "1.5.2",
+  "version": "1.6.0",
   "description": "C++ bindings for V8",
   "keywords": [ "V8", "native", "addon", "module", "bindings" ],
+  "engines": { "node": ">=8.0.0" },
   "main": "include_dirs.js",
   "repository": { "type": "git", "url": "git://github.com/pmed/v8pp.git" },
   "author": "Pavel Medvedev <pmedvedev@gmail.com>",