Require libsodium 0.4.5 at a minimum

libsodium 0.4.5 fully exposes all of the cryptographic constants needed for the
new FFI bindings.
Update CI task to libsodium 0.4.5
Merge pull request #86 from cryptosphere/require-libsodium-0-4-5

Require libsodium 0.4.5 at a minimum