CMake: Place hard version dependency on >= 2.8.9

Apparently there's a huge difference between even minor versions of
cmake in terms of additional features.

Change-Id: Idade6466e815a7dfdeaad9080d05527ac1b0c97a
Reviewed-on: http://review.couchbase.org/43157
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>