Make dependency versions more flexible, add phpunit as dev dependency

By replacing `3.7.*` with `~3.7`, we'll allow users to have more recent versions of dependencies installed. All tests are passing with Monolog 1.7 and Guzzle 3.8.1, so the strict requirement doesn't seem needed.

I've also added PHPunit as a dev dependency, so we no longer need a global version installed, and also added the tests directory to phpunit.xml.dist to further simplify running tests. You can now simply run `vendor/bin/phpunit` or simply `phpunit` if installed globally.