npm: Lock mini-css-extract-plugin at version 2.4.5

2.5 has an unintended API breakage:

  webpack-contrib/mini-css-extract-plugin#896

Author: martinpitt

package.json

@@ -35,7 +35,7 @@
     "eslint-webpack-plugin": "^2.5.3",
     "htmlparser": "^1.7.7",
     "jed": "^1.1.1",
-    "mini-css-extract-plugin": "^2.4.5",
+    "mini-css-extract-plugin": "~2.4.5",
     "po2json": "^1.0.0-alpha",
     "qunit": "^2.9.3",
     "sass": "^1.35.2",