Edit changelog of 2.0 version to note a BC break

[Nek-]

This MR #51 (and this commit e305841 ) fix a serious issue, but also break in some circumstances.

After discussion with @mcsky if we should rollback the change on v2.0 or simply notice it in the changelog, we decided that it's an actual fix and previous usages were wrong. A note about how to fix it should be added. And here is the "quick fix" for those getting errors after upgrade:

class SimpleBCDictionary extends SimpleDictionary
{
    public function getKeys()
    {
        return array_map('strval', array_keys($this->values));
    }
}