Stellar-base

Released on April 17, 2019
- Upgraded some dependencies.

Released on April 1, 2019
- Small bug fixes.

Released on February 28, 2019
- Add include_failed parameter to `Horizon.account_operations`, `Horizon.account_transactions`, `Horizon.transactions`,
`Horizon.transaction_operations`, `Horizon.ledger_operations`, `Horizon.ledger_transactions`
and `Horizon.operations` for including failed transactions in the result.
- Add `Horizon.fee_stats` for getting useful information about fee stats in the last 5 ledgers.
- Add `Horizon.base_fee` for fetching the current base fee, `Builder` uses it to set the default fee.

Released on February 1, 2019
- Add offset parameter to `Horizon.trade_aggregations` to reflect new changes to the endpoint in horizon-0.15.0

Released on December 31, 2018
- Breaking change: Decode memo_text into bytes internally
- Bugfix: Raise HorizonError(status_code=http_reply_code) after multiple queries and still failing.

Released on December 12, 2018
- Remove numpy dependency for installation