Fix Travis builds to use tweetnacl

[djelenc]

Currently all travis builds are either with or without libsodium.

Since libzmq defaulted to tweetnacl, it may be useful to run Travis against that, too.

I had tried to configure such builds, but I failed: libsodium kept getting cloned, compiled, installed and linked against libzmq, regardless of the environment variable that I set. (I tried CURVE=libsodium, CURVE=tweetnacl, WITH_LIBSODIUM=0, without that variable and a few other combinations, but all Travis builds continued to use libsodium. )

I'm sure this post is more of a treatise on my incompetence than an actual bug report, but I think that building against defaults should definitely be part of the Travis build.

[benjamg]

This sounds sensible, I really need to keep up with changed to libzmq and update things here. As I understand it the tweetnacl should simplify things for us anyway and maybe let us reduce the overall number of build options.