Support for Slackware 32 & 64 bit to install-redis script

when using linux, i typically deploy to slackware instances but couldn't due to the distro checks in install-redis

tested the produced redis build itself (per redis quick start) against slackware 13.34 64bit and the most current slackware 14.1 32/64bit and seems fine. let me know if there are any more exhaustive tests that should be performed.