Corrected clean target.

Makefile

@@ -5,7 +5,7 @@ install:
 	chmod 755 /usr/local/bin/ssl-ca
 
 clean:
-	[ -f .server.pid ] && kill "$$(cat .server.pid)"
+	if [ -f .server.pid ]; then kill "$$(cat .server.pid)"; fi
 	rm -rf openssl.cnf certs keys CA.key CA.crt CA.p12 CA.srl .server.pid
 
 test: clean