Fix SyntaxError on middleware registration

We picked this up with the faraday-http-cache test suite, since the tests in this project the `faraday_middleware` file isn't loaded by any of the existing tests - let me know I should add a require for it on the `helper` file.

In related news, there is some loading issues related to `Faraday::Env` and other dependencies that is breaking the existing tests. I might check on that later :smile: