Require cmake version >= 3.17.

Upgrading from cmake version 3.13.1 made this error go away:

CMake Error at CMakeLists.txt:67 (install):
      install TARGETS given no RUNTIME DESTINATION for executable target
      "nethack".

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.12)
+cmake_minimum_required(VERSION 3.17)
 file(STRINGS "version.txt" NLE_VERSION)
 project(nle VERSION ${NLE_VERSION})
 message(STATUS "Building nle backend version: ${NLE_VERSION}")