Update CHANGELOG for the last two releases

Ohai 7.2.0 shipped with Chef 11.14.2
Ohai 7.2.2 was built with Chef 11.14.4 but wasn't released due to some missing commits in Chef
Ohai 7.2.4 was shipped with Chef 11.14.6
Merge pull request #402 from opscode/btm/fix-master-changelog

Update CHANGELOG for the last two releases