Freeze webpack-sources to 1.0.1.

This fixes compilation on Node 6.x.
Should be removed later when this bug is fixed in webpack-sources.
webpack/webpack-sources#27 (comment)

Author: RubenVerborgh

package.json

@@ -43,7 +43,8 @@
     "babel-preset-env": "^1.6.0",
     "babel-preset-es2015": "^6.24.1",
     "webpack": "^3.5.5",
-    "webpack-dev-server": "^2.7.1"
+    "webpack-dev-server": "^2.7.1",
+    "webpack-sources": "1.0.1"
   },
   "standard": {
     "globals": [