Restrict stone 0.2 to use cow < 0.6.0

cow 0.6 removed the ?templates argument which stone 0.2 depends on.

@Armael I hope this is OK. ?templates was removed from cow to decouple it from xmlm (now a dependency rather than included with cow). Will an XML tree traversal satisfy your use case?
Merge pull request #857 from dsheets/patch-11

Restrict stone 0.2 to use cow < 0.6.0