avoid the use of tempfile as long paths can make crontab barf

some crons will barf on long pathnames and this cannot be controlled when using ruby's tempfile library as it respects Dir.tmpdir

the simple answer is just *don't use tempfiles* and, instead, use a simple pipe to avoid the issue altogether

this will avoid issues like #412

note: https://github.com/javan/whenever/pull/418 is ***crazy*** as it would blow the top of the max command line length for even modestly sized crontabs