Update the dependencies versions

- Require node 6 at a minimum to support the newer ES features.  Node
  version functionality determined via https://node.green.
- Update dev dependencies to their latest versions.  Mocha 3.4 relied
  on a version of growl with a known vulnerability.
- Fix the keywords section, this field is actually an array.
- Update eslint syntax to ES6.

package.json

@@ -1,7 +1,9 @@
 {
   "name": "base58",
   "version": "1.0.1",
-  "keywords": "base58, flickr",
+  "keywords": [
+    "base58, flickr"
+  ],
   "description": "Flickr Flavored Base58 Encoding and Decoding",
   "licenses": [
     {
@@ -22,14 +24,14 @@
   },
   "main": "./src/base58",
   "engines": {
-    "node": ">= 4"
+    "node": ">= 6"
   },
   "devDependencies": {
-    "eslint": "^4.1.1",
-    "eslint-config-prettier": "^2.3.0",
-    "eslint-plugin-prettier": "^2.2.0",
-    "mocha": "^3.4.2",
-    "prettier": "^1.5.3"
+    "eslint": "^5.6.0",
+    "eslint-config-prettier": "^3.0.0",
+    "eslint-plugin-prettier": "^2.7.0",
+    "mocha": "^5.2.0",
+    "prettier": "^1.14.3"
   },
   "scripts": {
     "lint": "eslint . --fix",