Remove https-proxy-agent transitive dependency

[jonny-improbable]

browser-headers@0.4.1 /Users/jonny/Projects/js-browser-headers
└─┬ karma-browserstack-launcher@1.3.0
  ├─┬ browserstack@1.5.0
  │ └── https-proxy-agent@1.0.0 

[jonny-improbable]

We are waiting on karma-runner/karma-browserstack-launcher#135 to drive the update down the dependency tree.

[jonny-improbable]

Note that https-proxy-agent has been upgraded on master by including https-proxy-agent as a direct devDepdendency. I'll keep this ticket open until karma-browserstack-launcher publishes an updated version that updates https-proxy-agent as a transitive dependency of this project.