Fixes compatibility with Node.js 8.

[austinkelleher]

When using on Node 8, I receive the following error:

/Proj/node_modules/require-dir/index.js:97
            if (!require.extensions.hasOwnProperty(ext)) {
                                    ^

TypeError: require.extensions.hasOwnProperty is not a function
    at requireDir (/Proj/node_modules/require-dir/index.js:97:37)
    at loadOfficialExtensions (/Proj/node_modules/laravel-elixir/dist/bootstrap/LoadExtensions.js:15:27)
    at Object.<anonymous> (/Proj/node_modules/laravel-elixir/dist/bootstrap/LoadExtensions.js:30:1)
    at Module._compile (module.js:573:30)
    at Object.Module._extensions..js (module.js:584:10)
    at Module.load (module.js:507:32)
    at tryModuleLoad (module.js:470:12)
    at Function.Module._load (module.js:462:3)
    at Module.require (module.js:517:17)
    at require (internal/module.js:11:18)

This is because Node.js 8 does not supper hasOwnProperty on require.extension. After further inspection, this issue has been fixed in aseemk/requireDir#46. The only caveat to using the latest version of require-dir is that it dropped support for the already deprecated Node.js version 0.10 and 0.11. I'm not sure if you're still supporting these versions, but if so, it should be a major version bump.

/cc @JeffreyWay

[zhuangya]

i filed same PR in #723 three month ago, but no one is responding :(

[austinkelleher]

@zhuangya Thanks for following up about this. I took a brief look at the PRs to see if anyone else had opened this same PR, but I must of overlooked yours. I will go ahead and close out this issue in favor of yours.