Update README.md

[gjtorikian]

Faraday::UploadIO has been deprecated for some time; this PR simply updates the README example to use the new class.

[coveralls]

Pull Request Test Coverage Report for Build 7715866238

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 32 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.002%) to 89.169%

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Files with Coverage Reduction	New Missed Lines	%
lib/slack/real_time/stores/store.rb	4	77.98%
lib/slack/real_time/concurrency/async.rb	28	1.39%

Totals
Change from base Build 6727999731:	0.002%
Covered Lines:	5055
Relevant Lines:	5669

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💛 - Coveralls

[dblock]

@greggroth Thanks. Care to add a test that uses this class? This way we'd catch the deprecation next time.