Revert "Fix pushd usage in setup.sh"

This reverts commit af90e2c989664c76c78a6b24e135c370fa41a0a4.
Revert "[setup/libsodium] Update to latest version of libsodium."

This reverts commit 453e6c141434ce28482580ec054f6d87948321c2.
Revert "[build/setup.sh] Add libsodium setup to install flow. NOTE: for some reason we hang on pip install -e ."

This reverts commit 4f9e5e24b4cbde7effeea4e5c38dba335ca74668.
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
requirements.txt: Consistently use HTTPS to fetch code from GitHub
Don't ship with editable (-e) repos in requirements.txt

pip clones such repos into src/, and it prompts you if you try to change the
location of the remote:

    Obtaining zerorpc from git+https://github.com/inboxapp/zerorpc-python.git@master#egg=zerorpc (from -r requirements.txt (line 21))
      git clone in ./src/zerorpc exists with URL git://github.com/inboxapp/zerorpc-python.git
      The plan is to install the git repository https://github.com/inboxapp/zerorpc-python.git
    What to do?  (s)witch, (i)gnore, (w)ipe, (b)ackup Your response ('exit') was not one of the expected responses: s, i, w, b
    What to do?  (s)witch, (i)gnore, (w)ipe, (b)ackup Exception:
    Traceback (most recent call last):
      File "/usr/lib/python2.7/dist-packages/pip/basecommand.py", line 126, in main
        self.run(options, args)
      File "/usr/lib/python2.7/dist-packages/pip/commands/install.py", line 223, in run
        requirement_set.prepare_files(finder, force_root_egg_info=self.bundle, bundle=self.bundle)
      File "/usr/lib/python2.7/dist-packages/pip/req.py", line 930, in prepare_files
        req_to_install.update_editable(not self.is_download)
      File "/usr/lib/python2.7/dist-packages/pip/req.py", line 378, in update_editable
        vcs_backend.obtain(self.source_dir)
      File "/usr/lib/python2.7/dist-packages/pip/vcs/git.py", line 105, in obtain
        if self.check_destination(dest, url, rev_options, rev_display):
      File "/usr/lib/python2.7/dist-packages/pip/vcs/__init__.py", line 204, in check_destination
        response = ask('What to do?  %s' % prompt[0], prompt[1])
      File "/usr/lib/python2.7/dist-packages/pip/util.py", line 103, in ask
        response = raw_input(message)
    EOFError: EOF when reading a line

The git repos in src/ also don't get cleaned by git-clean, which is
slightly annoying.
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.

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