Increase PHPBrake version constraint to "^0.8.0" (#18)

I've recently released version [0.8.0 of PHPBrake](
https://github.com/airbrake/phpbrake/blob/master/CHANGELOG.md
). This version adds the new remote config feature to periodically fetch
the project's remote notifier settings and caches them to the system
temp dir. The [`remoteConfig` option]( https://github.com/airbrake/phpbrake#remoteconfig)
can control this feature. This adds PHPBrake to the list of notifiers
that support the remote config feature(Ruby, Go, Java, JavaScript, Python).
This increase to the version constraint to adds remote config support to
laravel-airbrake.

Co-authored-by: Theo Kouzelis <theo@wirebox.co.uk>

composer.json

@@ -4,7 +4,7 @@
  "keywords": ["airbrake", "laravel"],
  "require": {
  "php": ">=5.4",
- "airbrake/phpbrake": "^0.7.3",
+ "airbrake/phpbrake": "^0.8.0",
  "illuminate/support": "^5.0|^6.0|^7.0|^8.0|^9.0"
  },
  "license": "MIT",