Remove rake/rdoctask
Remove rake/rdoctask

Update to use rdoc/task instead of the obsolete rake/rdoctask currently there.

Related to: https://github.com/vmware/rbvmomi/issues/51

10xEngineer/rbvmomi,rcorrieri/rbvmomi,logankimmel/rbvmomi,10xEngineer/rbvmomi,omar-infinio/rbvmomi,vmware/rbvmomi