build(travis): rm nightly, add php7.4

Author: adhocore

.travis.yml

@@ -10,11 +10,7 @@ php:
   - 7.1
   - 7.2
   - 7.3
-  - nightly
-
-matrix:
-  allow_failures:
-    - php: nightly
+  - 7.4
 
 install:
   - composer install --prefer-dist