Permit versions of faraday_middleware up to v0.12.x
Permit versions of faraday_middleware up to v0.12.x

`faraday_middleware` has been updated, bringing a bunch of
new features and security fixes in v0.12.2.

Given that the gem hasn't hit a major version yet, locking the
dependencies up to the next minor version feels sensible to ensure
compatability.

gocardless/callcredit-ruby