[Gfycat] update json api endpoint

[JimmStout]

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Fixes #18333

Based on the suggestions on the issue page, I changed the URL for the JSON api, and now, the Gfycat tests are passing:

youtube-dl$ python test/test_download.py TestDownload.test_Gfycat
[Gfycat] DeadlyDecisiveGermanpinscher: Downloading video info
[info] Writing video description metadata as JSON to: test_Gfycat_DeadlyDecisiveGermanpinscher.info.json
[debug] Invoking downloader on u'https://fat.gfycat.com/DeadlyDecisiveGermanpinscher.mp4'
[download] Destination: test_Gfycat_DeadlyDecisiveGermanpinscher.mp4
[download] 100% of 10.00KiB in 00:00
.
----------------------------------------------------------------------
Ran 1 test in 1.832s

OK
youtube-dl$ python test/test_download.py TestDownload.test_Gfycat_1
[Gfycat] JauntyTimelyAmazontreeboa: Downloading video info
[info] Writing video description metadata as JSON to: test_Gfycat_1_JauntyTimelyAmazontreeboa.info.json
[debug] Invoking downloader on u'https://zippy.gfycat.com/JauntyTimelyAmazontreeboa.mp4'
[download] Destination: test_Gfycat_1_JauntyTimelyAmazontreeboa.mp4
[download] 100% of 10.00KiB in 00:00
.
----------------------------------------------------------------------
Ran 1 test in 1.902s

OK