fixes crash when selected "history" in preview of page being created

Hi,

While taking gollum for a spin i have encountered a crash 2nd:

Steps to reproduce;
1. Select "new" -> "Ok"
2. Select "preview"
3. Select "History"
4. crash on "@versions    = @page.versions :page => @page_num"

I have also removed "history" button when we are displaying preview of page that is being created as it serves no purpose.

Best Regards