Fix travis config

.travis.yml

@@ -18,7 +18,7 @@ matrix:
           env: deps=low
     fast_finish: true
     allow_failures:
-      - 7.4snapshot
+      - php: 7.4snapshot
 
 before_script:
   - echo "extension = redis.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini