Read api_key from environment variable.

I haven't tested this, but would be happy to; when I try to run the tests via `phpunit`, I get the following error:

```
PHP Warning:  require_once(/Users/josh/code/honeybadger-php/tests/../vendor/autoload.php): failed to open stream: No such file or directory in /Users/josh/code/honeybadger-php/tests/test_helper.php on line 3
```

It looks like I'm missing a vendor directory -- any ideas? Also, are you aware of how I could test this out in a local composer project before it has been published to packagist? (I'm looking for the equivalent of doing `gem 'honeybadger', path: '../honeybadger-ruby'` in a local Gemfile.)

Sorry for the n00b questions... It has been a while since I worked with PHP. :)