TST: Errors with Giles webhook

[pllim]

I think I know why #236 didn't see a Giles status. Example log: https://circleci.com/gh/OpenAstronomy/openastronomy.github.io/180

- ./_site/gsoc/background.html
  *  External link https://mirca.github.io/gsoc-application/ failed: 404 No error
- ./_site/gsoc/suborg_guidelines.html
  *  External link https://socis.esa.int failed: response code 0 means something's wrong.
             It's possible libcurl couldn't connect to the server or perhaps the request timed out.
             Sometimes, making too many requests at once also breaks things.
             Either way, the return message (if any) from the server is: SSL peer certificate or SSH remote key was not OK
- ./_site/members/index.html
  *  External link https://asaip.psu.edu/organizations/iaa/iaa-working-group-of-cosmostatistics/ failed: 404 No error
- ./_site/projects/2018/poliastro/reference-frames.html
  *  External link http://aa.usno.navy.mil/publications/docs/Circular_179.pdf failed: response code 0 means something's wrong.
             It's possible libcurl couldn't connect to the server or perhaps the request timed out.
             Sometimes, making too many requests at once also breaks things.
             Either way, the return message (if any) from the server is: Couldn't resolve host name
- ./_site/projects/2019/astropy/stcs_regions.html
  *  External link https://github.com/at88mph/opencadc_stc/tree/master/opencadc_stc failed: 404 No error
- ./_site/projects/2019/poliastro/validation-framework.html
  *  External link https://blog.poliastro.space/poliastro-benchmarks/ failed: 404 No error
htmlproofer 3.15.1 | Error:  HTML-Proofer found 6 failures!

[dpshelio]

I don't think it's to do with the broken links. I believe it's because I've not set it properly. It never worked (or at least that I've seen it).