Remove built-in libsodium
Mention that libsodium is now required
Update .travis.yml to compile and install libsodium
All the currently available GUIs only work with OpenDNS :(
The Windows GUI by OpenDNS looks even more abandoned than other GUIs.
Statically link libsodium for now
Merge branch 'ext-sodium'

* ext-sodium:
  Statically link libsodium for now
  Update .travis.yml to compile and install libsodium