require older CMake version to make Travis happy

CMakeLists.txt

 # Keystone Assembler Engine (www.keystone-engine.org)
 # By Nguyen Anh Quynh, 2016
 
-cmake_minimum_required(VERSION 2.8.12.2)
+cmake_minimum_required(VERSION 2.8.7)
 
 set(KEYSTONE_VERSION_MAJOR 1)
 set(KEYSTONE_VERSION_MINOR 0)

llvm/CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
-cmake_minimum_required(VERSION 2.8.12.2)
+cmake_minimum_required(VERSION 2.8.7)
 
 set(LLVM_INSTALL_TOOLCHAIN_ONLY ON)