fix: pin vue-loader to 15.6.3

Author: greenkeeper[bot]

package.json

@@ -50,7 +50,7 @@
     "stylelint-config-sass-guidelines": "^5.3.0",
     "stylelint-scss": "^3.5.1",
     "uglifyjs-webpack-plugin": "^2.1.1",
-    "vue-loader": "^15.5.1",
+    "vue-loader": "15.6.3",
     "vue-template-compiler": "^2.6.6",
     "webpackbar": "^3.1.4"
   },