Clean up build matrix and make code coverage work again

.travis.yml

@@ -1,26 +1,23 @@
 sudo: false
-language: php
-
 cache:
   directories:
     - $HOME/.composer/cache
 
+language: php
+
 php:
   - 7.2
   - 7.3
   - 7.4
   - nightly
 
 env:
-  global:
-    - CODE_COVERAGE=""
-  matrix:
-    - PREFER_LOWEST="--prefer-lowest --prefer-stable"
-    - PREFER_LOWEST=""
+  - PREFER_LOWEST="--prefer-lowest --prefer-stable"
+  - PREFER_LOWEST=""
 
 jobs:
   include:
-    - php: 7.3
+    - php: 7.4
       env:
         - PREFER_LOWEST=""
         - CODE_COVERAGE="--coverage-clover build/logs/clover.xml"
@@ -38,8 +35,4 @@ script:
   - if [[ $TRAVIS_PHP_VERSION < 7.4 ]] ; then composer run-script lint; fi;
 
 after_script:
-  - if [[ $CODE_COVERAGE != "" ]]; then php vendor/bin/php-coveralls -v; fi;
-
-matrix:
-  allow_failures:
-    - php: nightly
+  - if [[ $CODE_COVERAGE != '' ]]; then php vendor/bin/php-coveralls -v; fi;