Apply requested changes to Travis configuration

Author: timoschinkel

.travis.yml

@@ -10,6 +10,7 @@ php:
     - 7.0
     - 7.1
     - 7.2
+    - 7.3
     - hhvm
 
 env:
@@ -24,6 +25,9 @@ matrix:
     allow_failures:
         - php: hhvm
     fast_finish: true
+    include:
+        - php: 7.0
+          env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest" COVERAGE=true TEST_COMMAND="composer test-ci"
 
 before_install:
     - if [[ $COVERAGE != true && $TRAVIS_PHP_VERSION != hhvm ]]; then phpenv config-rm xdebug.ini; fi