Add compatibility to PHP 8

.travis.yml

@@ -30,6 +30,9 @@ matrix:
     - php: 7.2
     - php: 7.3
     - php: 7.4
+    - php: nightly
+      env:
+        - COMPOSER_DEV_STABILITY=true
 
 before_install:
   - sudo add-apt-repository ppa:mc3man/trusty-media -y
@@ -46,6 +49,10 @@ before_install:
     if [ "$COMPOSER_FLAGS" == "--prefer-lowest" ]; then
       composer require "roave/security-advisories" dev-master --no-update
     fi;
+  - |
+    if [ "$COMPOSER_DEV_STABILITY" == "true" ]; then
+      composer config minimum-stability dev
+    fi;
 
 install:
   - sudo apt-get install -y ffmpeg