build: run CI against lowest dependencies

Author: Douglas Greenshields

.travis.yml

@@ -6,7 +6,19 @@ php:
     - 7.3
     - 7.4
 
-before_script: composer install --dev --prefer-source
+matrix:
+    include:
+        - php: 7.1
+          env: deps=low
+
+env:
+    global:
+        - deps=high
+
+before_script:
+    - if [ "$deps" = "high" ]; then composer install --dev --prefer-source; fi;
+    - if [ "$deps" = "low" ]; then composer --prefer-lowest --prefer-stable --ignore-platform-reqs update; fi
+    - if [ -f $HOME/.phpenv/versions/$(phpenv global)/etc/conf.d/xdebug.ini ]; then phpenv config-rm xdebug.ini; fi
 
 script:
     - vendor/bin/phpunit -v

composer.json

@@ -23,7 +23,7 @@
         "symfony/console": "~2.1|~3.0",
         "guzzlehttp/guzzle": "~6.1",
         "phpunit/phpunit": "^7",
-        "phpstan/phpstan": "^0.12"
+        "phpstan/phpstan": "^0.12.5"
     },
     "suggest": {
         "symfony/console": "To use CLI OEmbed scripts.",