Temporarily build only PHP 8.0 to diagnose potential CI issues

.travis.yml

@@ -5,27 +5,15 @@ os: linux
 language: php
 
 php:
-  - 7.1
-  - 7.2
-  - 7.3
-  - 7.4
   - 8.0
-  - nightly
 
 cache:
   directories:
     - $HOME/.composer/cache
 
 env:
   jobs:
-    - PREFER_LOWEST="--prefer-lowest"
     - PREFER_LOWEST=""
-    - SSL_TEST="yes" SSL_CA="ssl/ca.pem" SSL_PEER_NAME="server.rmq"
-    - SSL_TEST="yes" SSL_CA="ssl/ca.pem" SSL_PEER_NAME="server.rmq" SSL_CLIENT_CERT="ssl/client.pem" SSL_CLIENT_KEY="ssl/client.key"
-
-jobs:
-  allow_failures:
-    - php: nightly
 
 install:
   - composer update --prefer-source $PREFER_LOWEST