Fix a bug in Router#url & append Router object to ctx.

[HeavenDuke]

I tried to fixed a bug found within Router#url under the latest version, where I get errors when trying to get a url with a name provided in an embedded router middleware, the change doesn't affect other parts(at least all test is passed :b ).
Also I find that in Koa2 you cannot access router object as previous version, where you can use:

router.get('book-detail', '/books/:book_id', function *() {
     // some code...
});
router.post('create-book', '/books', function *() {
    // some code...
    this.redirect(this.app.url('book-detail', {book_id: book.id}));
});

So I append the router object to ctx. This leads to the following usage:

router.get('book-detail', '/books/:book_id', function (ctx) {
    // some code...
});
router.post('create-book', '/books', function (ctx) {
    // some code...
    ctx.redirect(ctx.router.url('book-detail', {book_id: book.id}));
});

All test are passed under development environment.

[marvinhagemeister]

I'm curious as to why the test fail on node 4 + 5. I can't see anything not supported in older node versions with the changes in this PR.

[HeavenDuke]

@marvinhagemeister maybe it's because I used async/await(since generator function is deprecated in koa2.x) but it seems that async/await is only supported in lower version of nodejs.

[marvinhagemeister]

@HeavenDuke I'm confused. There is no async/await included in the changeset or did I miss something obvious?

[HeavenDuke]

It's just a guess, since the error log shows that the error arise from koa's lib/application.js, not from what I wrote.

[marvinhagemeister]

oh I see. That makes a lot of sense. Koa did drop support for older node versions with version 2 and requires a minimum of node v7.6.0. In this case I think we should drop travis testing for older node versions and align with the requirements of koa itself.

[HeavenDuke]

right. I have dropped tests for 4 and 5, now everything works fine.