Add nightly builds

.travis.yml

@@ -7,6 +7,7 @@ php:
   - 7.2
   - 7.3
   - 7.4
+  - nightly
 
 cache:
   directories:
@@ -18,7 +19,7 @@ matrix:
           env: deps=low
     fast_finish: true
     allow_failures:
-      - php: 7.4
+      - php: nightly
 
 before_script:
   - echo "extension = redis.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini