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, which will allow
libnacl to be built against libsodium >= 0.5.0.
Merge pull request #16 from terminalmage/fix-get_nacl

Fix _get_nacl() for libsodium >= 0.5.0