Increase dependency options

- Improves test document
    + PHP7.4
    + Lowest dependencies
    + Highest dependencies

.travis.yml

@@ -1,11 +1,24 @@
 language: php
 
-php:
-  - 7.2
-  - 7.3
+jobs:
+  include:
+    - stage: "PHP7.2 - lowest"
+      php: 7.2
+      script:
+        - composer update -n --prefer-dist  --prefer-lowest --no-suggest
+        - composer dump-autoload
+        - php vendor/bin/phpunit
 
-before_script:
-  - composer install --prefer-source --no-interaction
-  - composer dump-autoload
+    - stage: "PHP7.3 - highest"
+      php: 7.3
+      script:
+        - composer update -n --prefer-dist --no-suggest
+        - composer dump-autoload
+        - php vendor/bin/phpunit
 
-script: vendor/bin/phpunit
+    - stage: "PHP7.4 - highest"
+      php: 7.4
+      script:
+        - composer update -n --prefer-dist --no-suggest
+        - composer dump-autoload
+        - php vendor/bin/phpunit

composer.json

@@ -25,13 +25,13 @@
     ],
     "require": {
         "php": "^7.2",
+        "ext-json": "*",
+        "ext-pcntl": "*",
         "yosymfony/resource-watcher": "^2.0",
-        "symfony/console": "^4.3",
+        "symfony/console": "^4.3 || ^5.0",
         "react/event-loop": "^1.1",
-        "symfony/yaml": "^4.3",
+        "symfony/yaml": "^4.3 || ^5.0",
         "react/child-process": "^0.6.1",
-        "ext-json": "*",
-        "ext-pcntl": "*",
         "alecrabbit/php-cli-snake": "^0.5"
     },
     "autoload": {