Enable xdebug with travis environment variable

Author: iquito

.travis.yml

@@ -2,6 +2,7 @@ env:
     global:
         - CC_TEST_REPORTER_ID=62f7e6db79d2f390676a4b3c62a3bcb1d47011fa6a457abcc77481b7a842bc91
         - GIT_COMMITTED_AT=$(if [ "$TRAVIS_PULL_REQUEST" == "false" ]; then git log -1 --pretty=format:%ct; else git log -1 --skip 1 --pretty=format:%ct; fi)
+        - XDEBUG_MODE=coverage
 language: php
 php:
     - '7.2'