Let the CI runner know about builds that this build depends on

This allows us to implement artifacts passing: runner will download artifacts from all prior builds. It will happen automatically, and always, as long as artifacts are enabled.

## The changes:

This MR exposes list of prior builds in CI::API::Builds.

**The API response when asking for builds**

```json
{
  "id": 48584,
  "ref": "0.1.1",
  "tag": true,
  "sha": "d63117656af6ff57d99e50cc270f854691f335ad",
  "status": "success",
  "name": "pages",
  "token": "9dd60b4f1a439d1765357446c1084c",
  "stage": "test",
  "project_id": 479,
  "project_name": "test",
  "commands": "echo commands",
  "repo_url": "http://gitlab-ci-token:token@gitlab.example/group/test.git",
  "before_sha": "0000000000000000000000000000000000000000",
  "allow_git_fetch": false,
  "options": {
    "image": "docker:image",
    "artifacts": {
      "paths": [
        "public"
      ]
    },
    "cache": {
      "paths": [
        "vendor"
      ]
    }
  },
  "timeout": 3600,
  "variables": [
    {
      "key": "CI_BUILD_TAG",
      "value": "0.1.1",
      "public": true
    }
  ],
  "dependencies": {
    "builds": [
      {
        "id": 48584,
        "ref": "0.1.1",
        "tag": true,
        "sha": "d63117656af6ff57d99e50cc270f854691f335ad",
        "status": "success",
        "name": "build",
        "token": "9dd60b4f1a439d1765357446c1084c",
        "stage": "build",
        "project_id": 479,
        "project_name": "test",
        "artifacts_file": {
          "filename": "artifacts.zip",
          "size": 0
        }
      }
    ]
  }
}
```

## How it will work?

**Example:**
```
build:
  type: build
  script:
  - echo TEST > test_file
  artifacts:
    untracked: true

rspec:
  type: test
  script:
  - test-my-project

staging:
  type: deploy
  script:
  - scp test_file root@server.com:
```

**The flow:**
1. We run `build`. The `build` creates a file `test_file`. This file gets archived and send us build artifacts.
2. We run `rspec`. The `rspec` downloads build artifacts from `build`. Uses the `test_file`.
3. We run `staging`. The `staging` downloads build artifacts from `build` and `rspec`, but since the `rspec` doesn't have build artifacts we skip that build. Deploys the `test_file`.

This partially implements the https://gitlab.com/gitlab-org/gitlab-ce/issues/3423.

In the next release we will introduce option to configure what artifacts are received.

/cc @grzesiek @DouweM @sytse @rspeicher


See merge request !2437