api: revisions aren't sorted

[yarikoptic]

which I think would be better to sort them so folks wouldn't even rely on sorting "null" and semantic versions

https://openfmri.org/dataset/api/ds000009/:

    "revision_set": [
        {
            "revision_number": "1.1.0",
            "notes": "Update orientation information in NIFTI headers for better left/right determination",
            "date_set": "2016-02-21"
        },
        {
            "revision_number": "1.0.0",
            "notes": "Initial Release",
            "date_set": "2014-02-03"
        },
        {
            "revision_number": "2.0.0",
            "notes": "Dataset has been re-created and organized in BIDS format.",
            "date_set": "2016-03-25"

[chrisgorgo]

@rwblair (@yarikoptic accidentally posted to a wrong repo)