Update module version for node v8.x

V8 5.7 landed recently in node core, causing the module version to get bumped.

lib/extensions.js

@@ -69,7 +69,7 @@ function getHumanNodeVersion(abi) {
     case 47: return 'Node.js 5.x';
     case 48: return 'Node.js 6.x';
     case 51: return 'Node.js 7.x';
-    case 53: return 'Node.js 8.x';
+    case 54: return 'Node.js 8.x';
     default: return false;
   }
 }