Update dependency semver to v6 (#437)

Author: renovate[bot]

package-lock.json

@@ -49,7 +49,7 @@
     "mocha": "5.2.0",
     "portfinder": "1.0.20",
     "selenium-assistant": "5.4.0",
-    "semver": "5.6.0",
+    "semver": "6.0.0",
     "sinon": "7.3.0",
     "which": "1.3.1"
   },