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:

    pyca/pynacl#79

requirements.txt

@@ -54,6 +54,6 @@ enum==0.4.4
 git+https://github.com/inboxapp/inbox-python.git@master#egg=inbox-python
 -e git://github.com/inboxapp/inbox-python.git@master#egg=inbox-python
 #pynacl==0.2.3
--e git://github.com/kav-ya/pynacl.git@libsodium_fix#egg=pynacl
+git+https://github.com/kav-ya/pynacl.git@libsodium_fix#egg=pynacl
 pyaml==14.05.7
 enum==0.4.4