Remove XDebug from PHP runtime within Travis CI environment.
Remove XDebug from PHP runtime within Travis CI environment.

Uses the snippet from the Travis CI Docs found here:

[https://docs.travis-ci.com/user/languages/php#Disabling-preinstalled-PHP-extensions](https://docs.travis-ci.com/user/languages/php#Disabling-preinstalled-PHP-extensions)

Fixes #3812

rodrigoprimo/wp-cli,aaemnnosttv/wp-cli,aaemnnosttv/wp-cli,GaryJones/wp-cli,2ndkauboy/wp-cli,miya0001/wp-cli,aaemnnosttv/wp-cli,gitlost/wp-cli,rodrigoprimo/wp-cli,trepmal/wp-cli,gitlost/wp-cli,2ndkauboy/wp-cli,2ndkauboy/wp-cli,gitlost/wp-cli,rodrigoprimo/wp-cli,GaryJones/wp-cli,trepmal/wp-cli,GaryJones/wp-cli,trepmal/wp-cli,miya0001/wp-cli,miya0001/wp-cli