Updated the changelog to reflect the serenity-core repo. For Bintray this is a bit of a hack, since the BinTray serenity-core package gets artifacts from two repos, serenity-core and serenity-maven-plugin, which are separate only because of the fact that core needs to be built and deployed before the maven plugin generation task in the serenity-maven-plugin build can be done. So the changelogs will be up-to-date on Github for both repos, but the one on bintray will only reflect core.