Add Node 8 to the support versions lists

This is a workaround for nodejs/citgm#389. The install/build flow
needs to be updated so that unsupported environments result in an
informative warning instead of a failure.

lib/extensions.js

@@ -69,6 +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';
     default: return false;
   }
 }