chore(package): update gulp-typescript to version 4.0.1

Closes #50

Author: greenkeeper[bot]

package.json

@@ -44,7 +44,7 @@
     "gulp-task-graph-visualizer": "^1.0.0",
     "gulp-tsfmt": "^1.0.2",
     "gulp-tslint": "^7.0.1",
-    "gulp-typescript": "^3.1.3",
+    "gulp-typescript": "^4.0.1",
     "gulp-util": "^3.0.7",
     "jsdom": "^9.8.3",
     "jsx-chai": "^4.0.0",