Update CHANGELOG for PR #3100 [skip ci]

Author: ponylang-main

CHANGELOG.md

@@ -14,6 +14,7 @@ All notable changes to the Pony compiler and standard library will be documented
 
 ### Changed
 
+- Stop Supporting Ubuntu Artful ([PR #3100](https://github.com/ponylang/ponyc/pull/3100))
 - Upgrade Docker image to use LLVM 7.0.1 ([PR #3079](https://github.com/ponylang/ponyc/pull/3079))
 - Stop trying to be clever when finding user's LLVM installation ([PR #3077](https://github.com/ponylang/ponyc/pull/3077))
 - Stop supporting Ubuntu Trusty ([PR #3076](https://github.com/ponylang/ponyc/pull/3076))