add new .travis.yml for linter

.travis.yml

@@ -1,10 +1,16 @@
+sudo: required
+dist: trusty
 language: php
 php:
   - '5.6'
   - '7.0'
 before-script:
   - composer install
   - composer update
-script: composer update && ./vendor/bin/phpunit --bootstrap vendor/autoload.php tests/
+script:
+  - composer update && ./vendor/bin/phpunit --bootstrap vendor/autoload.php tests/
+  - if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then git clone https://github.com/Jhut89/Mailchimp-API-3.0-PHP-travis.git travis; fi
+  - if [ "$TRAVIS_PULL_REQUEST" != "false" ] && [ ! -d "./travis" ]; then exit 1; fi
+  - if [ -d "./travis" ]; then sudo chmod +x ./travis/* && ./travis/build_script.sh; fi
 matrix:
   fast_finish: true