libvmaf: install models

[barrbrain]

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In ffmpeg, the default model path is hardcoded: /usr/local/share/model/vmaf_v0.6.1.pkl
Once the models are installed in this formula, we may follow up by injecting the actual path in the ffmpeg formula

[carlocab]

Thanks!

[BrewTestBot]

@carlocab has triggered a merge.