Travis: don't allow PHP 7.4 build to fail

As PHP 7.4 has been released, the build against PHP 7.4 should no longer be allowed to fail.

As that build was combined with the PHPStan check which - for now - should be allowed to fail, I've split the build into two:
1. a "normal" PHP 7.4 build which is not allowed to fail.
2. a PHPStan specific build which is allowed to fail.

.travis.yml

@@ -22,6 +22,8 @@ matrix:
       env: PHPUNIT_INCOMPAT=1
     - php: 7.3
       env: PHPUNIT_INCOMPAT=1
+    - php: "7.4snapshot"
+      env: PHPUNIT_INCOMPAT=1
     - php: "7.4snapshot"
       env: PHPSTAN=1 PHPUNIT_INCOMPAT=1
       addons:
@@ -38,6 +40,7 @@ matrix:
 
   allow_failures:
     - php: "7.4snapshot"
+      env: PHPSTAN=1 PHPUNIT_INCOMPAT=1
     - php: nightly
 
 before_install: