Improve CI testing

removed 'snapshot' prefix from php 8.0
added nighlty php version testing with allowing to fail

.travis.yml

@@ -4,7 +4,13 @@ php:
   - 7.2
   - 7.3
   - 7.4
-  - 8.0snapshot
+  - 8.0
+  - nightly
+
+matrix:
+    fast_finish: true
+    allow_failures:
+        - php: nightly
 
 # This triggers builds to run on the new TravisCI infrastructure.
 # See: http://docs.travis-ci.com/user/workers/container-based-infrastructure/
@@ -23,4 +29,4 @@ script:
   - vendor/bin/phpunit --coverage-clover=coverage.xml
 
 after_script:
-  - bash <(curl -s https://codecov.io/bash)
+  - bash <(curl -s https://codecov.io/bash)