SDK needs to be updated to handle Horizon 0.17.0 breaking changes

[ire-and-curses]

In Horizon release 0.17.0 (released 26 Feb 2019), several breaking changes were made (changelog), changing or removing fields that had previously been marked as deprecated.

Breaking changes

Fields removed in this version:
    Root > protocol_version, use current_protocol_version and core_supported_protocol_version.
    Ledger > transaction_count, use successful_transaction_count and failed_transaction_count.
    Signer > public_key, use key.
This Horizon version no longer supports Core <10.0.0. Horizon can still ingest version <10 ledgers.
Error event name during streaming changed to error to follow W3C specification.

Here is the relevant Horizon commit, showing the fields that were changed: stellar/go@575707e

This is an urgent issue! Any code using this SDK to access a Horizon 0.17.0 server (which includes the SDF public horizon instance at https://horizon.stellar.org/) will be broken until the SDK is updated to match the new response fields.