Fix _get_nacl() for libsodium >= 0.5.0

libsodium 0.5.0 resulted in a SONAME bump to libsodium.so.10, so libnacl fails
to load it. This fixes that issue, and will allow libnacl to be built
against libsodium >= 0.5.0.

**NOTE: This will require that we cut a new libnacl release, so that it can be
built and added to EPEL.**