Use single brackets instead of double in common.sh

On Ubuntu, I get the following errors when running the examples:
stderr: scripts/parrot-server.sh: 3: scripts/common.sh: [[: not found
stderr: scripts/parrot-feeder.sh: 3: scripts/common.sh: [[: not found

According to http://stackoverflow.com/questions/9666102/bash-double-bracket-issue, the sh command on Ubuntu does not support double brackets. Using single brackets fixes the problem.