Share data between rake tasks with ENV

Rake tasks can be loaded in any order, so don't depend upon global
variables which require specific ordering to be set. Environment
variables are a much better approach to sharing data between rake tasks.
Remove custom rake task for continuous integration

Since all testing now compiles a local copy of libsodium and loads the
library from it, there is no need for a custom task to do this for the
continuous integration server. It just happens automagically anyway.
Rename sodium rake tasks to libsodium

These tasks really have to do with the libsodium library, so should be
named accordingly.