Merge pull request soroco#9 from mkai/patch-2

Fixed demo script not compatible with Bash < 4.2 (important for macOS).

demo.sh

@@ -23,7 +23,7 @@ YELLOW='\033[1;33m'
 GRAY='\033[1;30m'
 RESET='\033[0m'
 
-if [ -v "PYTHON" ]
+if [ -n "${PYTHON+x}" ]
 then
     printf "${GRAY}Using Python: ${PYTHON}\n"
 else