Add PHP 8.0 to the Travis CI matrix, use 'nightly' builds (#462)

Author: sodabrew

Diff for: .travis.yml

@@ -2,7 +2,8 @@ dist: xenial
 
 language: php
 php:
-    - master
+    - nightly
+    - 8.0
     - 7.4
     - 7.3
     - 7.2
@@ -12,7 +13,7 @@ php:
 matrix:
     fast_finish: true
     allow_failures:
-        - php: master
+        - php: nightly
 
 env:
     - LIBMEMCACHED_VERSION=1.0.18 # Debian Jessie / Ubuntu Xenial