Update dependency selenium-assistant to v5.4.0 (#416)

Author: renovate[bot]

package-lock.json

@@ -48,7 +48,7 @@
     "mkdirp": "0.5.1",
     "mocha": "5.2.0",
     "portfinder": "1.0.20",
-    "selenium-assistant": "5.3.0",
+    "selenium-assistant": "5.4.0",
     "semver": "5.6.0",
     "sinon": "7.2.3",
     "which": "1.3.1"