Revert "Up minimum cmake version to 3.5."

This reverts commit 7ffd0d8.

Author: jonsimantov

CMakeLists.txt

@@ -14,7 +14,7 @@
 
 # Top level CMake file that defines the entire Firebase C++ SDK build.
 
-cmake_minimum_required (VERSION 3.5)
+cmake_minimum_required (VERSION 3.1)
 
 set (CMAKE_CXX_STANDARD 14)
 set (CMAKE_CXX_STANDARD_REQUIRED YES)  # Don't fall back to an earlier version.