Fix 'error: sodium.h: No such file or directory' while installing pynacl

This removes -e (`pip install --editable`) from the pynacl line in
requirements.txt, and uses git+https:// because pip will break if the
URI isn't in the format `vcs+protocol://...`

This may be related to the following pynacl issue:

    https://github.com/pyca/pynacl/issues/79

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

Differential Revision: https://review.inboxapp.com/D394
requirements.txt: Consistently use HTTPS to fetch code from GitHub

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