Clarifies that the endIndex for slice isn't included. 

Other JS references clarify this and I found myself tripping on it today since it wasn't obvious.
Fix typo in `unordered list`
[BUGFIX beta] Ensure {{link-to}} path observers are reregistered after render.

Prior to this change we called `registerObserver` on `init` so that `this.rerender` was triggered
when the link text changes (see [here](https://github.com/emberjs/ember.js/blob/3a7f4ae7c0be07f064f31e1df3f06d38ff866f4c/packages/ember-routing/lib/helpers/link_to.js#L192-L196)).

This worked properly to trigger the first `rerender`, but since `registerObserver` tears down the observer on `willClearRender`
(see [here](https://github.com/emberjs/ember.js/blob/3a7f4ae7c0be07f064f31e1df3f06d38ff866f4c/packages/ember-views/lib/views/view.js#L2273-2275)) and the observers
were not set back up, we would only handle a single change.

This change ensures that the observers are setup after each time the view is rerendered.
Merge pull request #3930 from CodingFu/patch-1

Fix typo in `unordered list`
Merge pull request #3928 from MiguelMadero/document-slice

Clarifies that the endIndex for slice isn't included.
Merge pull request #3932 from rjackson/fix_link_to_non-block

[BUGFIX beta] Ensure {{link-to}} path observers are reregistered after render.
[BUGFIX beta] Do not attempt to serialize undefined models.

If a routes model is undefined do not attempt to serialize it.

Previously, this would cause an error like: `Cannot call get with 'id'
on an undefined object.`.
Remove broken Vagrantfile - Closes #3938
remove cookbooks because we don't have vagrant
Remove `.vagrant` from gitignore

Vagrant itself was removed in bd4ee53f1aee1888a9468dc04af7fbe3039264fa
Merge pull request #3941 from speedmanly/patch-1

Remove `.vagrant` from gitignore
Merge pull request #3933 from rjackson/fix_serialize_on_missing_models

[BUGFIX beta] Do not attempt to serialize undefined models.
Remove Vagrant references from CONTRIBUTING.md.
Merge pull request #3939 from fivetanley/remove-cookbooks

remove cookbooks because we don't have vagrant
Merge pull request #3942 from rjackson/remove_vagrant_from_contrib

Remove Vagrant references from CONTRIBUTING.md.
PromiseProxy prefers Promise.cast over resolve, this makes it work correctly with promise inheritance.
Merge pull request #3943 from stefanpenner/promise-proxy-uses-cast

PromiseProxy prefers Promise.cast over resolve, this makes it work corre...