fix: configure composer to use a GitHub personal access token

Author: childish-sambino

.travis.yml

@@ -1,6 +1,7 @@
 language: php
 
 before_script:
+  - composer config -g github-oauth.github.com "$GIT_HUB_TOKEN"
   - composer install
   - if [ "$dependencies" = "lowest" ]; then composer update --prefer-lowest --prefer-stable -n; fi;