No longer need to set PATH for workhorse development

No longer needed because of:

https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/57

See merge request !181

Add docs about overriding host with host file

This MR adds a note to `set-up-gdk.md` about using `./host` file to override host the workhorse process will bind to.

This is especially useful when someone uses GDK inside the Docker container and wants to expose Workhorse.

See merge request !180

Small refactor of set-up-gdk.md



See merge request !179

Update documentation to use Ruby 2.3.1

GitLab is using Ruby 2.3.1 now on master, I figure the documentation should also be updated.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4948

cc: @zj @jacobvosmaer-gitlab 

See merge request !178

update docs



See merge request !176

Closes:
https://gitlab.com/gitlab-org/gitlab-development-kit/issues/145

Add Vagrant links

cc: @jacobvosmaer-gitlab 

See merge request !175

More documentation cross-links



See merge request !172

Add 'gdk' command wrapper and gem

Combine `make` and `./run` into `gdk`. Can be invoked as `gdk`, e.g. `gdk run db`.

This does not remove `make` and `./run`, it just adds an integrated wrapper.

Installation become:

```
gem install gitlab-development-kit
gdk init # clones repo
cd gitlab-development-kit
gdk install gitlab_repo=https://bla
```

See merge request !174

Use a GOPATH to install gitlab-workhorse

This allows you to use more Go tools during development.

See merge request !173