Update sass-loader (#9)

Sass loader on npm install automatically updates to latest version, and
there's a known issue at [1]. Fix this by downgrading sass-loader.

Links:
1. vuejs/vue-cli#4513

Author: fredlawl

package-lock.json

@@ -19,7 +19,7 @@
         "popper.js": "^1.12",
         "resolve-url-loader": "^2.3.1",
         "sass": "^1.24.4",
-        "sass-loader": "^8.0.0",
+        "sass-loader": "^7.1.0",
         "vue": "^2.6.11",
         "vue-template-compiler": "^2.6.11"
     }