Fix Advert test model to inherit from ClusterableModel.
Fix Advert test model to inherit from ClusterableModel.

django-modelcluster 1.0 includes a system check to ensure that ParentalKey points to a ClusterableModel; Advert falls foul of this.

gogobook/wagtail,hanpama/wagtail,mikedingjan/wagtail,nimasmi/wagtail,iansprice/wagtail,Tivix/wagtail,quru/wagtail,takeflight/wagtail,mixxorz/wagtail,JoshBarr/wagtail,rsalmaso/wagtail,davecranwell/wagtail,kaedroho/wagtail,kurtrwall/wagtail,gasman/wagtail,inonit/wagtail,hamsterbacke23/wagtail,nimasmi/wagtail,nilnvoid/wagtail,kurtw/wagtail,timorieber/wagtail,chrxr/wagtail,wagtail/wagtail,serzans/wagtail,Tivix/wagtail,mixxorz/wagtail,FlipperPA/wagtail,hanpama/wagtail,nealtodd/wagtail,nrsimha/wagtail,inonit/wagtail,nilnvoid/wagtail,FlipperPA/wagtail,thenewguy/wagtail,zerolab/wagtail,Tivix/wagtail,mayapurmedia/wagtail,zerolab/wagtail,kurtrwall/wagtail,zerolab/wagtail,thenewguy/wagtail,FlipperPA/wagtail,timorieber/wagtail,nimasmi/wagtail,rsalmaso/wagtail,Toshakins/wagtail,kaedroho/wagtail,nealtodd/wagtail,inonit/wagtail,zerolab/wagtail,mikedingjan/wagtail,wagtail/wagtail,iansprice/wagtail,wagtail/wagtail,rsalmaso/wagtail,Toshakins/wagtail,takeflight/wagtail,kurtrwall/wagtail,iansprice/wagtail,rsalmaso/wagtail,gasman/wagtail,JoshBarr/wagtail,davecranwell/wagtail,chrxr/wagtail,jnns/wagtail,torchbox/wagtail,mayapurmedia/wagtail,gogobook/wagtail,thenewguy/wagtail,inonit/wagtail,Tivix/wagtail,nilnvoid/wagtail,hanpama/wagtail,nilnvoid/wagtail,nrsimha/wagtail,nutztherookie/wagtail,gasman/wagtail,JoshBarr/wagtail,mixxorz/wagtail,hamsterbacke23/wagtail,JoshBarr/wagtail,mixxorz/wagtail,mikedingjan/wagtail,gogobook/wagtail,kaedroho/wagtail,nealtodd/wagtail,mayapurmedia/wagtail,nutztherookie/wagtail,wagtail/wagtail,serzans/wagtail,mayapurmedia/wagtail,kurtw/wagtail,zerolab/wagtail,serzans/wagtail,takeflight/wagtail,rsalmaso/wagtail,jnns/wagtail,mixxorz/wagtail,gasman/wagtail,kurtw/wagtail,gogobook/wagtail,thenewguy/wagtail,nrsimha/wagtail,torchbox/wagtail,kurtw/wagtail,davecranwell/wagtail,thenewguy/wagtail,quru/wagtail,timorieber/wagtail,nutztherookie/wagtail,nrsimha/wagtail,nutztherookie/wagtail,davecranwell/wagtail,takeflight/wagtail,hanpama/wagtail,wagtail/wagtail,iansprice/wagtail,nealtodd/wagtail,Toshakins/wagtail,chrxr/wagtail,torchbox/wagtail,hamsterbacke23/wagtail,jnns/wagtail,Toshakins/wagtail,hamsterbacke23/wagtail,serzans/wagtail,kurtrwall/wagtail,FlipperPA/wagtail,torchbox/wagtail,gasman/wagtail,kaedroho/wagtail,nimasmi/wagtail,quru/wagtail,kaedroho/wagtail,jnns/wagtail,quru/wagtail,timorieber/wagtail,mikedingjan/wagtail,chrxr/wagtail