Replace HTTPoison with Tesla for flexibility

[OldhamMade]

Changes

• Replace use of HTTPoison with Tesla
• Update tests & fixtures to expect httpc as the base HTTP library (the default for Tesla)
• Bump exvcr version

Resolves #96

[coveralls]

Pull Request Test Coverage Report for Build cb61475a0e6a1c5473f3629c51cf78d57767e082-PR-97

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 80.142%

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Totals
Change from base Build 3506210596ec6f2ef7ac531d66519b6ae05dc4a3:	0.7%
Covered Lines:	113
Relevant Lines:	141

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💛 - Coveralls

[floriank]

sorry for taking so long in reviewing this, I am a bit busy and moving the repo to contentful-userland has apparently changed notification settings.

Will try ASAP :)

[floriank]

@OldhamMade Could you rebase/merge latest master? 💚

[floriank]

Thank you - this completely went under as I've been swamped with projects during work time and never received a notification for the status checks to pass (which seem to have hung themselves after the repo was moved to another organization)