Force HTTP/1.1 in HTTPSFuture until HTTP/2 support stabilizes

Summary:
Fixes T13391. See that task for more detailed discussion.

Recent versions of cURL default to HTTP/2 on some connections but do not support HTTP/2 proxies, so connecting to certain correctly configured servers can leave you dead in the water.

Force connections down to HTTP/1.1 for now.

Test Plan:

• Created this revision to establish I didn't break anything catastrophically.
• See task for evidence this actually works.

Maniphest Tasks: T13391

Differential Revision: https://secure.phabricator.com/D20744