[build/setup.sh] Add libsodium setup to install flow.

Summary:
Workaround for the "error: sodium.h: No such file or directory" bug in
pynacl and/or cffi: https://github.com/pyca/pynacl/issues/79

Fixes https://github.com/inboxapp/inbox/issues/95

Test Plan: `vagrant up --provision` from scratch

Reviewers: charles

Reviewed By: charles

Subscribers: mg

Differential Revision: https://review.inboxapp.com/D635