Use PHPSpec branch alias instead of dev-master in composer.json

When I tried to run `composer install` in my project I got the error:
```
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - phpspec/phpspec dev-master requires phpspec/prophecy ~1.0.5@dev -> no matching package found.
    - henrikbjorn/phpspec-code-coverage 1.0.x-dev requires phpspec/phpspec dev-master -> satisfiable by phpspec/phpspec[dev-master].
    - Installation request for henrikbjorn/phpspec-code-coverage 1.0.*@dev -> satisfiable by henrikbjorn/phpspec-code-coverage[1.0.x-dev].
```

This project `composer.json` listed *PHPSpec* in `dev-master`, which has a branch alias `2.0.*-dev`, and *PHPSpec* depends on *Prophecy* using other branch alias (`1.5.*-dev`).

Seems that composer was not fetching the *Prophecy* branch alias when we don't use the *PHPSpec* branch alias.

To reproduce this issue, just remove your vendor folder and install this extension without this pull request.

This pull request fix it by changing *PHPSpec* dependency from `dev-master` to its alias `2.0.*@dev`