Sadly actual version data is not available from CSOM. You can get the data by extracting it from /_layouts/Versions.aspx?list={[THE LIST ID]}&ID=[THE ITEMID]&IsDlg=1;
I used HTMLAgilityPack to parse out the data I was looking for (a past value for a specific field)
HtmlDocument doc = new HtmlDocument();
doc.LoadHtml(DownloadedPageContent);
HtmlNodeCollection n = doc.DocumentNode.SelectNodes("//tr[@id='vv" + VersionId + FieldName +"']/td");
String value = n[1].InnerText.Trim();