Fix prerequisites for captains-log (#2688)

* Fix prerequisites for captains-log

We have strings in this exercise!

* Update config.json

Author: SleeplessByte

config.json

@@ -432,7 +432,10 @@
         ],
         "prerequisites": [
           "numbers",
-          "arithmetic-operators"
+          "arithmetic-operators",
+          "strings",
+          "objects",
+          "functions"
         ]
       }
     ],