Added laravel 5.6 to travis build

Author: korridor

.travis.yml

@@ -7,15 +7,15 @@ language: php
 matrix:
     include:
         - php: 7.1
-          env: LARAVEL='5.7.*' TESTBENCH='3.7.*' COMPOSER_FLAGS='--prefer-lowest'
+          env: LARAVEL='5.6.*' TESTBENCH='3.6.*' COMPOSER_FLAGS='--prefer-stable'
+        - php: 7.2
+          env: LARAVEL='5.6.*' TESTBENCH='3.6.*' COMPOSER_FLAGS='--prefer-stable'
+        - php: 7.3
+          env: LARAVEL='5.6.*' TESTBENCH='3.6.*' COMPOSER_FLAGS='--prefer-stable'
         - php: 7.1
           env: LARAVEL='5.7.*' TESTBENCH='3.7.*' COMPOSER_FLAGS='--prefer-stable'
-        - php: 7.2
-          env: LARAVEL='5.7.*' TESTBENCH='3.7.*' COMPOSER_FLAGS='--prefer-lowest'
         - php: 7.2
           env: LARAVEL='5.7.*' TESTBENCH='3.7.*' COMPOSER_FLAGS='--prefer-stable'
-        - php: 7.3
-          env: LARAVEL='5.7.*' TESTBENCH='3.7.*' COMPOSER_FLAGS='--prefer-lowest'
         - php: 7.3
           env: LARAVEL='5.7.*' TESTBENCH='3.7.*' COMPOSER_FLAGS='--prefer-stable'
         - php: 7.1

composer.json

@@ -12,10 +12,10 @@
     "license": "MIT",
     "require": {
         "php": "^7.1|^7.2|^7.3",
-        "illuminate/support": "^5.7|^5.8|^6.0"
+        "illuminate/support": "^5.6|^5.7|^5.8|^6.0"
     },
     "require-dev": {
-        "orchestra/testbench": "^3.7|^3.8|^4.0",
+        "orchestra/testbench": "^3.6|^3.7|^3.8|^4.0",
         "phpunit/phpunit": "^7.0|^8.0",
         "friendsofphp/php-cs-fixer": "^2.15",
         "squizlabs/php_codesniffer": "^3.4"