ci(travis): use PECL to install XDebug

Author: felixfbecker

.travis.yml

@@ -31,8 +31,9 @@ jobs:
         # Fix ruby error https://github.com/Homebrew/brew/issues/3299
         - brew update
         - brew tap homebrew/homebrew-php
-        - brew install php71
-        - brew install homebrew/php/php71-xdebug
+        - brew install php@7.1
+        - brew link --force --overwrite php@7.1
+        - pecl install xdebug-2.6.0
         - php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
         - php composer-setup.php
         - ln -s "`pwd`/composer.phar" /usr/local/bin/composer