Update travis

Author: norkunas

.travis.yml

@@ -16,6 +16,10 @@ matrix:
         # Test minimum supported dependencies with the latest and oldest PHP version
         - php: 7.1
           env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest"
+        - php: 7.2
+          env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest"
+        - php: 7.3
+          env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest"
         - php: 7.4
           env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest"