chore(travis): Set PHP 7.4 testing to stable version (#73)

.travis.yml

@@ -18,7 +18,7 @@ matrix:
     - php: 7.1
     - php: 7.2
     - php: 7.3
-    - php: 7.4snapshot
+    - php: 7.4
 
 before_install:
   # Speed up build time by disabling Xdebug when its not needed.