chore(package): update core-js to version 2.5.6

Author: greenkeeper[bot]

package.json

@@ -57,7 +57,7 @@
     "codelyzer": "4.0.0",
     "commitplease": "3.2.0",
     "conventional-github-releaser": "2.0.2",
-    "core-js": "2.5.5",
+    "core-js": "2.5.6",
     "css-loader": "0.28.11",
     "cssnano": "3.10.0",
     "del": "3.0.0",