Declare a direct dependency on http-proxy-agent

[cysp]

I'm seeing this error when running the latest version of danger-js:

internal/modules/cjs/loader.js:582
    throw err;
    ^

Error: Cannot find module 'http-proxy-agent'

… it looks like we used to get the package via a transitive dependency via @octokit/rest, but that changed with the recent upgrade of that package:
0859032#diff-8ee2343978836a779dc9f8d6b794c3b2L719

[DangerCI]

Danger run resulted in 2 warnings and 2 markdowns; to find out more, see the checks page.

Generated by 🚫 dangerJS against 5ab78ab

[orta]

Fixes #801

[orta]

Thanks!

[peril-staging]

Thanks for the PR @cysp.

This PR has been shipped in v7.0.1 - CHANGELOG.