Tr sets must be quoted

`Tr` sets (in square brackets) must be quoted. Otherwise the behaviour is unpredictable, e.g.:

### machine1:

    echo 'ABCDEFGHIJKL' | tr [A-Z] [a-z]
    aaaaaaaaaaaa

### machine2:

    echo 'ABCDEFGHIJKL' | tr [A-Z] [a-z]
    abcdefghijkl