update travis config

.travis.yml

@@ -5,6 +5,7 @@ php:
   - '7.0'
   - '7.1'
   - '7.2'
+  - '7.3'
   - nightly
 
 # cache vendor dirs
@@ -15,7 +16,7 @@ cache:
 
 install:
   - travis_retry composer self-update && composer --version
-  - travis_retry composer install --prefer-dist --no-interaction
+  - travis_retry composer install --no-interaction
 
 before_script:
   - sudo apt-get install p7zip-full