cmake: require 2.8.6

Change-Id: I08101d0f50503b4fb06c56bb926ddb0b713fa127
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 44808ac..3311b01 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -24,7 +24,7 @@
 # ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
 
-cmake_minimum_required(VERSION 2.6)
+cmake_minimum_required(VERSION 2.8.6)
 
 project(PT)