Add macos to Continuous Integration

[Sean P. Kelly]

If I extend the CI in #660 to build on macos, the integration tests fail:

 failures:

---- http_integ::test_retries stdout ----
thread 'http_integ::test_retries' panicked at 'Failed to run integration test HTTP servers, is docker running?: Failed to find our configured proxy 'toxictuf'

Caused by:
    I/O error: "error sending request for url (http://127.0.0.1:8472/proxies/toxictuf): error trying to connect: tcp connect error: Connection refused (os error 61)"', tough/tests/http.rs:131:14
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

It seems that the noxious-server isn't starting, but we don't capture enough output to determine why.