Update to Guzzle 6#4

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smreichling wants to merge 7 commits intoMCProHosting:masterfrom

Commits on Aug 7, 2015

Merge pull request #1 from quizlet/use-json-body
karoun
committed

Commits on Dec 16, 2015

Commits on Apr 25, 2016

Bump Guzzle to ~6.1
Peter Bakkum
committed
Merge pull request #3 from quizlet/guzzle-bump
Peter Bakkum
committed
Fix issues created by guzzle 6 transition
Peter Bakkum
committed
Merge pull request #4 from quizlet/guzzle-bump
Peter Bakkum
committed