FutureWarnings in epidata code

[patricklnz]

In getCaseData line 662, 719 FutureWarning: Passing literal json to 'read_json' is deprecated and will be removed in a future version. To read from a literal string, wrap it in a 'StringIO' object. In getVaccinationData line 146: FutureWarning: Setting an item of incompatible dtype is deprecated and will raise an error in a future version of pandas. In getVaccinationData line 520 FutureWarning: Series.__getitem__ treating keys as positions is deprecated.