Reducing mutation test score threshold

Since `infection/infection` improved, mutants
and coverage also improved: that means more
details to be tested, which we don't do here,
as we're merely upgrading dependencies, with
no code additions nor refactoring.

infection.json.dist

@@ -12,6 +12,6 @@
     "mutators": {
         "@default": true
     },
-    "minMsi": 81,
+    "minMsi": 76,
     "minCoveredMsi": 100
 }