GM: now that we use MD5 instead of CityHash, the JSON should say so

gm/tests/outputs/compared-against-different-pixels-json/output-expected/json-summary.txt

 {
    "actual-results" : {
       "failed" : {
          "565/selftest1" : {
-            "bitmap-cityhash" : 12927999507540085554
+            "bitmap-64bitMD5" : 12927999507540085554
          },
          "8888/selftest1" : {
-            "bitmap-cityhash" : 1209453360120438698
+            "bitmap-64bitMD5" : 1209453360120438698
          }
       },
       "failure-ignored" : null,
       "no-comparison" : null,
       "succeeded" : null
    },
    "expected-results" : {
       "565/selftest1" : {
-         "allowed-bitmap-cityhashes" : [ 8863920166200910451 ],
+         "allowed-bitmap-64bitMD5s" : [ 8863920166200910451 ],
          "ignore-failure" : false
       },
       "8888/selftest1" : {
-         "allowed-bitmap-cityhashes" : [ 13451349865803053525 ],
+         "allowed-bitmap-64bitMD5s" : [ 13451349865803053525 ],
          "ignore-failure" : false
       }
    }
 }