CMake: Update minimum required CMake version because

the LLVM integration requires tar.xz decompression support
which was added in 3.1

CMakeLists.txt

@@ -11,7 +11,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-cmake_minimum_required (VERSION 2.8)
+cmake_minimum_required (VERSION 3.1)
 
 set(NGRAPH_INCLUDE_PATH
     ${CMAKE_CURRENT_SOURCE_DIR}/src