Modified Redis recipe to add option to use Redis 2.6 or 2.8

Summary of changes

* changed redis version to a parameter loaded from attributes
* installs Redis 2.6.16 by default (available on both Gentoo 2009 and 12.11)  instead of Redis 2.4
* added the `override_hardmask true` parameter when installing Redis. This is needed when installing 2.8 on Gentoo 12.11
* added the default Redis 2.6 and 2.8 configuration files, merged with the EY sensible defaults
* added the hz variable (introduced in 2.6) to the Redis config
* added section in README.md on how to install a different version of Redis

Tests done
* Boot a new 12.11 util instance named redis. Result: util instance runs Redis 2.6.16
* Edit recipe to specify version 2.8.13, then upload and apply. Result: util instance has Redis 2.8.13 installed (needs a monit restart to run the new version)