Doesn't make an "empirical_frequencies" key when doing AA for phyml

    mus taxtastic/testfiles ‹master› » cat phyml_aa_stats.txt.json
    {
      "datatype": "AA", 
      "subs_model": "LG", 
      "program": "PhyML v3.0_export", 
      "ras_model": "gamma", 
      "gamma": {
        "alpha": 0.626, 
        "n_cats": 4
      }
    }

This makes pplacer choke.