Add CHANGELOG entry about the fix in loader
[[#261](https://github.com/ethereum/evmc/issues/261),
[#263](https://github.com/ethereum/evmc/pull/263)]
The `vmtester` tool now builds with MSVC with `/std:c++17`.
+- Fixed:
+ [[#305](https://github.com/ethereum/evmc/issues/305),
+ [#306](https://github.com/ethereum/evmc/pull/306)]
+ A loaded VM with incompatible ABI version is not properly destroyed.
## [6.2.2] - 2019-05-16
