Update dependency tslint to ~5.20.0

Author: renovate[bot]

package.json

@@ -66,7 +66,7 @@
     "gulp-svgstore": "^6.1.1",
     "material-design-icons": "^3.0.1",
     "ts-node": "~4.1.0",
-    "tslint": "~5.9.1",
+    "tslint": "~5.20.0",
     "typescript": "2.7.2"
   }
 }