Version bump to v0.7.1

Merge pull request #255 from alpacahq/fix-data-v2-issues

Quick fixes to remove calls to market data v1 from alpaca SDK

Merge pull request #237 from alpacahq/bump-version

- bump alpaca-trade-api to v1.2.0
- update pylivetrader version to 0.5.5
- update  pylivetrader docker image to v0.5.5

update version to 0.5.4

Update _version.py

update alpaca-trade-api to 0.51.0

Merge pull request #195 from alpacahq/change_can_trade_check

Process pool should be initiated once

update dockerfile to v0.5.0

added comment for migrationo part