Mackerel.client

* Use Faraday v1.0 54 (onk)

Breaking change

Now mackerel-client raises `Mackerel::Error` when HTTP requests failed.
Previously various exceptions (mainly `RuntimeError` or `NoMethodError`) happened in such cases.

* Use Faraday::Response::RaiseError middleware 49 (onk)

* fix unintended initializing http headers 47 (y_uuki)

* Use Rspec v3.x 43 (onk)



RELEASE
=======

1. Create release branch. (`git checkout -b version-<VERSION>`)
1. Bump version `VERSION` and `lib/mackerel/version.rb` files.
1. Write changes in CHANGELOG.md
1. Push release branch to upstream(GitHub). (`git push -u origin version-<VERSION>`)
1. Create Pull-Request on Github.
1. Merge Pull-Request to master branch on Github.
1. Tagging and Release. (`rake release`)