chore(package): update sinon to version 7.3.0

Closes #44

Author: greenkeeper[bot]

package.json

@@ -14,7 +14,7 @@
     "got": "^9.0.0",
     "mocha": "^5.2.0",
     "nyc": "^12.0.2",
-    "sinon": "^6.0.1"
+    "sinon": "^7.3.0"
   },
   "homepage": "https://github.com/gucong3000/postcss-pie#readme",
   "keywords": [