Fix tests failing on Ruby 1.8
Fix tests failing on Ruby 1.8

Tempfile doesn't automatically do string coercion when passed as an argument to File.
I've switched to using the `File#path` method explicitely.

Refs #845

timo-p/resque,bhighley/resque,hoffmanc/resque,ryan-drake-stoker/resque,coupa/resque,sideci-sample/sideci-sample-resque,begriffs/resque,tylerdooling/resque,jonatas/resque,jacknguyen/resque,kangkot/resque,delectable/resque,fmitech/resque,ejlangev/resque,resque/resque,coupa/resque,begriffs/resque,stitchfix/resque,igorjancic/resque,tylerdooling/resque,rutikasb/resque,Shopify/resque,CloudVLab/resque,resque/resque,ejlangev/resque,eileencodes/resque,lukeasrodgers/resque,Shopify/resque,ychaim/resque,m0dd3r/resque,vladimirich/resque,resque/resque,jacknguyen/resque,paran0ids0ul/resque,ducthanh/resque,xiangzhuyuan/resque,igorjancic/resque,l4u/resque,stvp/resque,vietnt/resque,stvp/resque,partap/resque,sufery/resque,CloudVLab/resque,paran0ids0ul/resque,stitchfix/resque,sideci-sample/sideci-sample-resque,Shopify/resque,lukeasrodgers/resque,AlexVPopov/resque,m0dd3r/resque,timo-p/resque,xiangzhuyuan/resque,sufery/resque,l4u/resque,vladimirich/resque,fmitech/resque,coupa/resque,lileeyao/resque,CloudVLab/resque,ngpestelos/resque,ktheory/resque,lileeyao/resque,delectable/resque,bonekost/resque,xiangzhuyuan/resque,hoffmanc/resque,redhotpenguin/resque,ktheory/resque,stvp/resque