Update test matrix

Author: TysonAndre

.travis.yml

@@ -1,13 +1,10 @@
 language: php
 
 php:
-  - 7.0
-  - 7.1
-  - 7.2
-  - 7.3
-  - 7.4
-  # Should be changed to 8.0 once travis officially provides that label.
-  - nightly
+  - '7.2'
+  - '7.3'
+  - '7.4'
+  - '8.0'
 
 env:
   - VALIDATION=false
@@ -30,12 +27,7 @@ cache:
 
 before_script:
   - if [[ $STATIC_ANALYSIS = true ]]; then composer require phpstan/phpstan --no-update; fi
-  - |
-    if php -r 'exit(PHP_MAJOR_VERSION < 8 ? 0 : 1);';
-      then composer install
-    else
-      composer install --ignore-platform-reqs
-    fi
+  - composer install
   - set -e # Stop on first error.
   - phpenv config-rm xdebug.ini || true
   - if find . -name "*.php" -path "./src/*" -path "./experiments/*" -path "./tools/*" -path "./syntax-visualizer/server/src/*" -exec php -l {} 2>&1 \; | grep "syntax error, unexpected"; then exit 1; fi