fix(package.json): Upgrade the dependencies to the latest

Upgraded Webpack loaders and fs-extra

BREAKING CHANGE: react-dev-utils is incompatible with the old one, therefore it is containing a
potential breaking change.

Author: halfzebra

package.json

@@ -32,29 +32,30 @@
     "cross-spawn": "^6.0.5",
     "css-loader": "^0.28.9",
     "dotenv": "^5.0.0",
-    "elm": "^0.18.0",
+    "elm": "~0.18.0",
     "elm-hot-loader": "0.5.4",
     "elm-test": "^0.18.12",
     "elm-webpack-loader": "^4.5.0",
     "file-loader": "^1.1.11",
-    "fs-extra": "^5.0.0",
+    "fs-extra": "^6.0.1",
     "html-webpack-plugin": "^3.0.7",
     "http-proxy-middleware": "^0.17.4",
-    "mini-css-extract-plugin": "^0.2.0",
+    "mini-css-extract-plugin": "^0.4.0",
     "minimist": "1.2.0",
     "postcss-flexbugs-fixes": "^3.3.0",
-    "postcss-loader": "2.1.0",
+    "postcss-loader": "2.1.5",
     "prompt": "1.0.0",
-    "react-dev-utils": "^5.0.0",
+    "react-dev-utils": "6.0.0-next.3e165448",
     "react-error-overlay": "^4.0.0",
     "string-replace-loader": "^2.1.1",
-    "style-loader": "^0.20.3",
+    "style-loader": "^0.21.0",
     "sw-precache-webpack-plugin": "^0.11.5",
     "uglifyjs-webpack-plugin": "^1.2.4",
     "url-loader": "^1.0.1",
     "webpack": "^4.2.0",
     "webpack-dev-server": "^3.1.1",
-    "webpack-manifest-plugin": "^2.0.0-rc.2"
+    "webpack-manifest-plugin": "^2.0.3",
+    "whatwg-fetch": "2.0.4"
   },
   "devDependencies": {
     "babel-eslint": "^8.2.1",