daniel.rb: fix RuboCop warnings about single-quoted strings.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Rewrite loop on one line.

This also addresses two RuboCop issues.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Clean up main program call.

Switch to using $MAIN_PROGRAM instead of $0 and move it to a guard clause
instead of a block.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Use fail instead of raise.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>