Test PHP 7.3 and 7.4 (#205)

* Test PHP 7.3 and 7.4

* Update .travis.yml

* Update .travis.yml

Author: barryvdh

.travis.yml

@@ -5,6 +5,8 @@ php:
   - 7.0
   - 7.1
   - 7.2
+  - 7.3
+  - 7.4snapshot
 
 # This triggers builds to run on the new TravisCI infrastructure.
 # See: http://docs.travis-ci.com/user/workers/container-based-infrastructure/
@@ -28,7 +30,9 @@ matrix:
       env: setup=lowest symfony="^3"
     - php: 7.1
       env: setup=lowest symfony="^4"
-
+  allow_failures:
+    - php: 7.4snapshot
+    
 install:
   - if [[ $symfony != '*' ]]; then travis_retry composer require symfony/http-foundation:${symfony} --no-update --no-interaction; fi
   - if [[ $setup = 'basic' ]]; then travis_retry composer install --prefer-dist --no-interaction; fi