chore(package): update gulp-exec to version 4.0.0 (#158)

Author: greenkeeper[bot]

package.json

@@ -51,7 +51,7 @@
     "gulp-autoprefixer": "^6.0.0",
     "gulp-clean-css": "^4.0.0",
     "gulp-eslint": "^5.0.0",
-    "gulp-exec": "^3.0.2",
+    "gulp-exec": "^4.0.0",
     "gulp-header": "^2.0.5",
     "gulp-less": "^4.0.1",
     "gulp-mocha": "^6.0.0",