fix: Updates package.json to _actual_ supported (tested) NodeJS versi…

…ons.

- 6 support was dropped in May (6718428), 4 and 5 were dropped in November 2018 (1d46627)

package.json

@@ -19,7 +19,7 @@
     "url": "git+https://github.com/conventional-changelog/standard-version.git"
   },
   "engines": {
-    "node": ">=4.0"
+    "node": ">=8.0"
   },
   "keywords": [
     "conventional-changelog",