Fix permil encoding

[CommonClimate]

The recent fix of #482 to address #473 seems to have improved things on the PC side but broke them on the Mac side. Double-quotes seem to work better than single quotes, so I updated metadata.yml accordingly.

To test the new code:

import pyleoclim as pyleo
pyleo.utils.available_dataset_names()
for name in ['LR04','EDC-dD','GISP2','cenogrid_d18O','cenogrid_d13C']:
    ts = pyleo.utils.load_dataset(name)
    ts.plot()

should yield something like this: