fix: Remove unused polyfills and update build config

Removes polyfills for `Object.assign` and `Set` which, as far as I can tell, are unused in the
codebase. Updates `webpack` from 3 to 4. Replaces `babel-preset-es2015` with `babel-preset-env`.
Reduces the minified bundle size (`feather.min.js`) by **9.3kB** (72.4kB to 63.1kB). Does not change
any functionality of the library.

package.json

@@ -27,7 +27,8 @@
     ]
   },
   "dependencies": {
-    "classnames": "^2.2.5"
+    "classnames": "^2.2.5",
+    "core-js": "^2.5.3"
   },
   "devDependencies": {
     "@commitlint/cli": "^5.2.5",
@@ -35,12 +36,10 @@
     "algoliasearch": "^3.27.1",
     "babel-cli": "^6.24.1",
     "babel-loader": "^7.1.1",
-    "babel-plugin-transform-object-rest-spread": "^6.26.0",
-    "babel-preset-es2015": "^6.24.1",
-    "babel-register": "^6.24.1",
+    "babel-preset-env": "^1.7.0",
+    "babel-preset-stage-2": "^6.24.1",
     "cheerio": "^1.0.0-rc.2",
     "commitizen": "^2.9.6",
-    "core-js": "^2.5.3",
     "cz-conventional-changelog": "^2.1.0",
     "eslint": "^4.15.0",
     "eslint-config-airbnb-base": "^12.1.0",
@@ -56,7 +55,8 @@
     "rimraf": "^2.6.2",
     "semantic-release": "^12.2.2",
     "svgo": "^0.7.2",
-    "webpack": "^3.10.0"
+    "webpack": "^4.8.3",
+    "webpack-cli": "^2.1.3"
   },
   "repository": {
     "type": "git",