Fixed cmake and Travis

Travis uses libsodium 0.4.2 instead of head of the master branch of [the git repository](https://github.com/jedisct1/libsodium), which may be unstable.

The Windows build assumes that the `sodium` folder from [libsodium 0.4.2](https://download.libsodium.org/libsodium/releases/libsodium-win32-0.4.2.tar.gz) is contained in the repository root.