Add a composer.lock file to freeze dep versions and added correct alias for prismic-kit

Add a composer.lock file to freeze dep versions and added correct alias for prismic-kit. The installed dep versions are the following;

```bash
Loading composer repositories with package information
Installing dependencies (including require-dev)
  - Installing symfony/event-dispatcher (v2.5.0)
    Loading from cache

  - Installing guzzle/guzzle (v3.9.1)
    Loading from cache

  - Installing prismic/php-sdk (dev-master a37b91e)
    Cloning a37b91ede025885ba1ef6fd335b03688fdc8c72a

  - Installing doctrine/lexer (v1.0)
    Loading from cache

  - Installing doctrine/annotations (v1.1.2)
    Loading from cache

  - Installing symfony/translation (v2.5.0)
    Loading from cache

  - Installing symfony/templating (v2.5.0)
    Loading from cache

  - Installing symfony/stopwatch (v2.5.0)
    Loading from cache

  - Installing symfony/security-core (v2.5.0)
    Loading from cache

  - Installing symfony/security-csrf (v2.5.0)
    Loading from cache

  - Installing symfony/routing (v2.5.0)
    Loading from cache

  - Installing symfony/filesystem (v2.5.0)
    Loading from cache

  - Installing psr/log (1.0.0)
    Loading from cache

  - Installing symfony/debug (v2.5.0)
    Loading from cache

  - Installing symfony/http-foundation (v2.5.0)
    Loading from cache

  - Installing symfony/http-kernel (v2.5.0)
    Loading from cache

  - Installing symfony/config (v2.5.0)
    Loading from cache

  - Installing symfony/dependency-injection (v2.5.0)
    Loading from cache

  - Installing symfony/framework-bundle (v2.5.0)
    Loading from cache
```