fix(package): update sinon to version 7.3.1

Closes #36

Author: greenkeeper[bot]

package.json

@@ -35,7 +35,7 @@
     "node-sass": "^4.5.3",
     "postcss-loader": "^2.0.6",
     "sass-loader": "^6.0.6",
-    "sinon": "^3.2.1",
+    "sinon": "^7.3.1",
     "style-loader": "^0.23.1",
     "webpack": "^3.5.5"
   }