CompatHelper: bump compat for AdvancedHMC to 0.8, (keep existing compat)

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This pull request changes the compat entry for the AdvancedHMC package from 0.5, 0.6 to 0.5, 0.6, 0.8. This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.