support laravel 6 and 7, remove composer.lock but add prefer lowest tests

test lowest dependencies

.gitignore

@@ -1,2 +1,3 @@
 vendor
 .idea
+composer.lock

.travis.yml

@@ -1,8 +1,20 @@
 language: php
+
 php:
-  - "7.4"
-  - "7.3"
-  - "7.2"
+  - 7.4
+  - 7.3
+  - 7.2
+
+env:
+  matrix:
+    - COMPOSER_FLAGS="--prefer-lowest"
+    - COMPOSER_FLAGS=""
+
+before_install:
+  - travis_retry composer self-update
+
 install:
-  - "composer self-update"
-  - "composer install"
+  - travis_retry composer update --prefer-source $PREFER_LOWEST
+
+script:
+  - phpunit

composer.json

@@ -16,10 +16,10 @@
   },
   "minimum-stability": "stable",
   "require": {
-    "illuminate/support": ">=5.2"
+    "illuminate/support": ">=5.2|^6.0|^7.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^8.0",
+    "phpunit/phpunit": "^8.0|^8.5",
     "mockery/mockery": "^1.3"
   },
   "extra": {