Fix 'error: sodium.h: No such file or directory' and related issues

- Revert "Fix pushd usage in setup.sh"
- Revert "[setup/libsodium] Update to latest version of libsodium."
- Revert "[build/setup.sh] Add libsodium setup to install flow. NOTE: for some reason we hang on pip install -e ."
- Fix 'error: sodium.h: No such file or directory' while installing pynacl
- requirements.txt: Consistently use HTTPS to fetch code from GitHub
- Don't ship with editable (-e) repos in requirements.txt

Test Plan:
- Destroy my vagrant VM, then run "vagrant up" to re-provision it.

Reviewers: charles, kav-ya

Differential Revision: https://review.inboxapp.com/D394