[CMake] Bump CMake minimum version to 3.13.4

This upgrade should be friction-less because we've already been ensuring
that CMake >= 3.13.4 is used.

This is part of the effort discussed on llvm-dev here:

  http://lists.llvm.org/pipermail/llvm-dev/2020-April/140578.html

Differential Revision: https://reviews.llvm.org/D78648

GitOrigin-RevId: afa1afd4108d973e059e5f5ad68cf01efe7985da
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c1fcf45..35ca8bc 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1 +1 @@
-cmake_minimum_required(VERSION 3.1)
+cmake_minimum_required(VERSION 3.13.4)
diff --git a/acxxel/CMakeLists.txt b/acxxel/CMakeLists.txt
index 547dd62..7e71446 100644
--- a/acxxel/CMakeLists.txt
+++ b/acxxel/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.1)
+cmake_minimum_required(VERSION 3.13.4)
 
 option(ACXXEL_ENABLE_UNIT_TESTS "enable acxxel unit tests" ON)
 option(ACXXEL_ENABLE_MULTI_DEVICE_UNIT_TESTS "enable acxxel multi-device unit tests" OFF)