Guard against tcp-backlog being set on < 2.8.5

Ubuntu trusty installs Redis 2.8.4 and the tcp-backlog config option did
not exist until 2.8.5.

Now it only sets it when the version is >= 2.8.5.
Merge pull request #1 from nickjj/patch

Guard against tcp-backlog being set on < 2.8.5