add php 7.3 to travis (webmozarts#84)

* add php 7.3 to travis

* disable xdebug for 7.3

.travis.yml

@@ -25,13 +25,14 @@ matrix:
     - php: 7.1
       env: COMPOSER_FLAGS='--prefer-lowest --prefer-stable'
     - php: 7.2
+    - php: 7.3
   allow_failures:
     - php: hhvm
     - php: nightly
   fast_finish: true
 
 before_install:
-  - if [[ $TRAVIS_PHP_VERSION != hhvm && $COVERAGE != yes ]]; then phpenv config-rm xdebug.ini; fi;
+  - if [[ $TRAVIS_PHP_VERSION != hhvm && $TRAVIS_PHP_VERSION != 7.3 && $COVERAGE != yes ]]; then phpenv config-rm xdebug.ini; fi;
   - if [[ $TRAVIS_REPO_SLUG = webmozart/assert ]]; then cp .composer-auth.json ~/.composer/auth.json; fi;
 
 install: composer update $COMPOSER_FLAGS --prefer-dist --no-interaction