Update ddtrace to 0.20.4

[pyup-bot]

This PR updates ddtrace from 0.20.1 to 0.20.4.

Changelog

0.20.4

This is a bug fix release, no code changes are required.

In this release we have fixed a bug that caused some configuration values to not get updated when set.

Changes
Bug fixes
* [bug] Integration config keys not being updated (816)

Read the [full changeset](https://github.com/DataDog/dd-trace-py/compare/v0.20.3...v0.20.4) and the [release milestone](https://github.com/DataDog/dd-trace-py/milestone/37?closed=1).

0.20.3

This is a bug fix release that requires no changes.

This release includes a fix for context propagation with `futures`. Under the right conditions we could incorrectly share a trace context between multiple `futures` threads which result in multiple traces being joined together in one.

Changes
Bug fixes
* [core] Allow futures to skip creating new context if one doesn't exist (806)

Read the [full changeset](https://github.com/DataDog/dd-trace-py/compare/v0.20.2...v0.20.3) and the [release milestone](https://github.com/DataDog/dd-trace-py/milestone/37?closed=1).

0.20.2

No changes are needed to upgrade to `0.20.2`.

This big fix release includes changes to ensure we properly read the HTTP response body from the trace agent before we close the HTTP connection.

Changes
Bug fixes

- [core] Call HTTPResponse.read() before HTTPConnection.close() (800)

Improvements
- [tests] limit grpcio version to >=1.8.0,<1.18.0 (802)
- [tools] Add confirmation to 'rake pypi:release' task (791 )

Read the [full changeset](https://github.com/DataDog/dd-trace-py/compare/v0.20.1...v0.20.2) and the [release milestone](https://github.com/DataDog/dd-trace-py/milestone/36?closed=1).

Links

• PyPI: https://pypi.org/project/ddtrace
• Changelog: https://pyup.io/changelogs/ddtrace/
• Repo: https://github.com/DataDog/dd-trace-py

[coveralls]

Pull Request Test Coverage Report for Build 678

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 44.861%

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Totals
Change from base Build 639:	0.0%
Covered Lines:	323
Relevant Lines:	720

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💛 - Coveralls

[pyup-bot]

Closing this in favor of #268