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.

.babelrc

 {
-  "presets": ["es2015"],
-  "plugins": ["transform-object-rest-spread"]
+  "presets": [
+    [
+      "env",
+      {
+        "targets": {
+          "browsers": ["last 2 versions"]
+        }
+      }
+    ],
+    "stage-2"
+  ]
 }

bin/build.sh

@@ -21,5 +21,5 @@ mkdir dist/icons
 ./node_modules/.bin/babel-node bin/build-svgs.js
 
 # Build JavaScript library
-./node_modules/.bin/webpack --output-filename feather.js
-./node_modules/.bin/webpack --output-filename feather.min.js -p
+./node_modules/.bin/webpack --output-filename feather.js --mode development
+./node_modules/.bin/webpack --output-filename feather.min.js --mode production

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",

webpack.config.babel.js → webpack.config.js

-import path from 'path';
+const path = require('path');
 
-export default {
-  entry: [
-    'core-js/fn/array/from',
-    'core-js/fn/object/assign',
-    'core-js/fn/set',
-    path.resolve(__dirname, 'src/index.js'),
-  ],
+module.exports = {
+  entry: ['core-js/fn/array/from', path.resolve(__dirname, 'src/index.js')],
   output: {
     path: path.resolve(__dirname, 'dist'),
     libraryTarget: 'umd',