Remove market data v1 calls and Polygon integration

⚠️ THIS RELEASE IS A BREAKING CHANGE ⚠️

We're still pre 1.x so I wont bump the major version but this release contains changes that are breaking such as removal of the polygon integration.

This release contains the following PRs:

• Remove market data v1 calls and Polygon integration (#35)

Update alpaca-trade-api

• Update the python-sdk to v0.52.0
• optmize api call for list_assets
• update readme doc

Enable Github Actions CI for testing

Run the package tests on the github CI (Ubuntu, python 3.6)

Limit iexfinance version

iexfinance 0.5.0 requires python 3.7 or above.
We still support 3.6, so we limit the version.

IEX Cloud API & Alpaca Pricing Source

Until now, IEX has been the primary source of data for pipeline-live. With their update to IEX Cloud, however, several of the data endpoints they provide will no longer be freely available for most use cases due to their monthly messaging limits. This update allows users to access IEX Cloud if they wish by setting an IEX_TOKEN environment variable, but also provides access to Alpaca's price data API, which does not impose messaging limits as IEX Cloud does. Please see the Readme for updated instructions on using the Alpaca pricing data source.