Replace bootstrap tarball
Add correct bootstrap-2.3.1.zip
Remove unnecessary parentheses in SQL query

These make SQLite 3.7.16.2 crash.

http://hydra.nixos.org/build/4815315
hydra-module.nix: find sendmail in /var/setuid-wrappers
hydra-module.nix: strip trailing whitspace
hydra-queue-runner: don't clutter the system log with debug message

Avoid the frequently printed

  hydra-queue-runner[10293]: system type `x86_64-linux': 2 active, 2 allowed, starting 0 builds

message. That information is only interesting when some build are
actually started.