Set the `minimum-stability` to `dev` for the master branch.

Not sure if this has been discussed before, but it would be nice if contributors could run:

    git clone git@github.com:sebastianbergmann/phpunit.git
    cd phpunit
    composer install --dev

But you'll get the following errors:

    Problem 1
      - The requested package phpunit/php-timer >=1.1.0 could not be found.
    Problem 2
      - The requested package sebastian/diff could not be found in any version, there may be a typo in the package name.
    Problem 3
      - The requested package sebastian/exporter could not be found in any version, there may be a typo in the package name.

The problem is that the master branch requires dev packages. Setting  `minimum-stability` to `dev` in `composer.json` fixes that problem.