Fix a build error during build_electron (#199)

It's a regression after 6c08eee.
We should use the external N-API for electron build, but it's failed to apply
the related patch because there is a difference in the code due to the upgrade
of node-addon-api.

ISSUE=#198

examples/electron/.node_addon_api_patch/disable-using-internal-n-api.patch

@@ -1,12 +1,13 @@
---- node_modules/node-addon-api/index.js	2017-08-26 03:40:40.000000000 +0900
-+++ node_modules_electron/node-addon-api/index.js	2017-10-08 22:32:13.000000000 +0900
+diff -urN node_modules/node-addon-api/index.js node_modules_electron/node-addon-api/index.js
+--- node_modules/node-addon-api/index.js	2017-11-22 20:11:18.000000000 +0900
++++ node_modules_electron/node-addon-api/index.js	2017-11-22 20:33:45.000000000 +0900
 @@ -14,8 +14,7 @@
  // We know which version of Node.js first shipped the incarnation of the API
  // available in *this* package. So, if we find that the Node.js version is below
  // that, we indicate that the API is missing from Node.js.
 -var isNodeApiBuiltin =
--  (versionArray[0] >= 8 && versionArray[1] >= 4 && versionArray[2] >= 0);
+-  (versionArray[0] > 8 || (versionArray[0] == 8 && versionArray[1] > 5));
 +var isNodeApiBuiltin = false;
 
- // So far it looks like even version 9 will need the flag. We need to adjust
- // this for the version where the flag is dropped whenever that version lands.
+ // The flag is not needed when the Node version is not 8, nor if the API is
+ // built-in, because we removed the flag at the same time as creating the final