fix(package): update semver to version 6.0.0

Closes #2616

package.json

@@ -24,7 +24,7 @@
     "ipaddr.js"             : "~1.9.0",
     "nopt"                  : "~4.0.1",
     "npid"                  : "~0.4.0",
-    "semver"                : "~5.6.0",
+    "semver"                : "~6.0.0",
     "sprintf-js"            : "~1.1.0",
     "haraka-config"         : ">=1.0.15",
     "haraka-constants"      : ">=1.0.5",