Allow http dependency v5+

[jeffgran-dox]

http version 5 is out, with the following breaking changes, according to their changelog:

• #576 #524 BREAKING CHANGE Preserve header names casing. ([@joshuaflanagan])

• #540 #538 BREAKING CHANGE Require explicit MIME type for Response#parse ([@ixti])

• #546 BREAKING CHANGE Provide initiating HTTP::Request object on HTTP::Response. ([@joshuaflanagan])

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• For the first: I searched for references to header and did not find anywhere we are relying on header case
• For the second: I searched for .parse and did not find anywhere we are calling Response#parse
• The third did break one test. Looking at the related PR, I saw this comment explaining why it was a breaking change:

  This is a breaking change for integrators like WebMock for example.
  Updating webmock fixed the test.

Therefore I believe it is safe to allow v5.