add min cmake version

Noticed errors in Build logs complaining of CMakeLists not specifying
minimum version.

Author: vimanyu

firestore/integration_test_internal/abseil-cpp.cmake

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+cmake_minimum_required(VERSION 2.8)
 include(ExternalProject)
 
 set(version 20200225)