Test catalogs do not pass validation

[gadomski]

As I was playtesting #151, I discovered that several of our test catalogs do not pass validation, e.g.:

(stactools) ➜  stactools git:(feature/stac-validate) stac validate tests/data-files/catalogs/planet-example-1.0.0-beta.2/collection.json
Validation failed for COLLECTION at tests/data-files/catalogs/planet-example-1.0.0-beta.2/collection.json with ID planet-disaster-data against schema at https://schemas.stacspec.org/v1.0.0/collection-spec/json-schema/collection.json
'2017-08-28T10:00:00-08:00' does not match '(\\+00:00|Z)$'

Failed validating 'pattern' in schema['allOf'][0]['properties']['extent']['properties']['temporal']['properties']['interval']['items']['items']:
    {'format': 'date-time',
     'pattern': '(\\+00:00|Z)$',
     'type': ['string', 'null']}

On instance['extent']['temporal']['interval'][0][0]:
    '2017-08-28T10:00:00-08:00'

tests/data-files/catalogs/test-case-1/catalog.json fails as well, and I'm sure there's more. We should add test cases that ensure that all of our test data files are valid.