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Make transformer ignore code with invalid filename

The transformFn should handle missing or non-string filenames since the
filename argument to vm.runInThisContext is optional.
Update hook test for Node 0.11 compatibility

Node 0.11 throws an exception if the second argument to runInThisContext
is not an object.
Merge pull request #215 from jason0x43/verify-filename

Hook transformer should verify filename argument