Update .travis.yml

Author: nicolus

.travis.yml

@@ -20,6 +20,7 @@ matrix:
 
 env:
   - COMPOSER_ALLOW_XDEBUG=0
+  - XDEBUG_MODE=coverage
 
 cache:
   directories:
@@ -36,4 +37,4 @@ script:
 
 after_script:
   - wget https://scrutinizer-ci.com/ocular.phar
-  - php ocular.phar code-coverage:upload --format=php-clover build/clover.xml
+  - php ocular.phar code-coverage:upload --format=php-clover build/clover.xml