bug when writing metadata

[alixdamman]

fix is:

def _dump_metadata(self, metadata):
    root = self.handle._handle.root
    root._v_attrs.metadata = metadata

[gdementen]

or...

def _dump_metadata(self, metadata):
    self.handle.root._v_attrs.metadata = metadata

[alixdamman]

Note: problem occurs for pandas < 0.18.0

[gdementen]

I guess adding a requirement for pandas >= 0.18 seems reasonable

[alixdamman]

It's actually worse than that. See PR #740