Fix a missing .tar.gz

tests/install_sodium.sh

 #!/bin/sh
 cd $HOME
 wget http://download.dnscrypt.org/libsodium/releases/libsodium-0.2.tar.gz
-tar xvf libsodium-0.2
+tar xvf libsodium-0.2.tar.gz
 cd libsodium-0.2
 ./configure --disable-debug --disable-dependency-tracking
 make