[FEATURE] Balances|Trustlines: Return ledger_index, use last validated ledger by default (RLJS-127)

Also, returns `validated` key as a boolean (from the rippled response)
I can do that as well! I'm still figuring out what the dev practices are for ripple-rest/lib. (Aside: That's technically a change in implementation unrelated to this PR. In my past experience, keeping commits atomic made reverting changes easier if they introduced additional bugs)