Clarify readme or simplify config regarding environments

[motin]

Nothing gets sent to sentry unless there is an array of enabledEnvironments and the environment configuration is set to a member of that array, ie:

  'components' => array(
    'sentry' => array(
      'class' => 'vendor.crisu83.yii-sentry.components.SentryClient',
      'dns' => '<replace-with-your-sentry-dns>',
      'environment' => 'foo',
      'enabledEnvironments' => array('foo'),
    ),
  ),

Neither "environment" nor "active environment" is part of Sentry-lingo, it seems to be something specific to one's application?

When you are at it, please add a comment that "dns" is the part that is sent as first argument to Raven_Client as per Sentry's official code examples.

Otherwise the setup was painless, nice!