symfony 4 has been released, remove stability=rc (#246)

Author: dbu

.travis.yml

@@ -40,7 +40,7 @@ matrix:
         - php: 7.2
           env: DEPENDENCIES="dunglas/symfony-lock:^3"
         - php: 7.2
-          env: DEPENDENCIES="dunglas/symfony-lock:^4" STABILITY="rc"
+          env: DEPENDENCIES="dunglas/symfony-lock:^4"
 
           # Latest commit to master
         - php: 7.2