update recent_changes file with info on __int64 fix
Changes to SUNDIALS since Release of May. 7, 2018.
+* Fixed Windows specific problem where sunindextype was not correctly
+ defined when using 64-bit integers for the sundials index type. On Windows
+ sunindextype is now defined as the MSVC basic type __int64.
+
* Updated the minimum required version of CMake to 2.8.12 and enabled
using rpath by default to locate shared libraries on OSX.
