[fix] add polyfill for node 6

Author: toxic-johann

index.js

@@ -1,6 +1,7 @@
 const shell = require('shelljs');
 const semver = require('semver');
 const { isFunction, isRegExp, isString, isEmpty } = require('toxic-predicate-functions');
+require('core-js/fn/object');
 
 class Helper {
   getPrefix(rawPrefix) {

package.json

@@ -24,6 +24,7 @@
   },
   "homepage": "https://github.com/toxic-johann/git-prefix-tag#readme",
   "dependencies": {
+    "core-js": "^2.5.3",
     "semver": "^5.5.0",
     "shelljs": "^0.8.1",
     "toxic-predicate-functions": "^0.1.5"