Make package Guzzle 7.x compatible

Author: miks

.gitignore

@@ -1,3 +1,4 @@
 .idea
+.phpunit.result.cache
 vendor
 coverage

.travis.yml

@@ -2,6 +2,7 @@ language: php
 
 php:
   - 7.4
+  - 8.0
 
 env:
   global:

composer.json

@@ -12,10 +12,11 @@
       "psr-4": { "Omnipay\\FirstDataLatvia\\" : "src/" }
     },
     "require": {
-      "omnipay/common": "~3.0"
+      "omnipay/common": "~3.0",
+        "php-http/guzzle7-adapter": "^1.0"
     },
     "require-dev": {
-      "omnipay/tests": "~3.0",
+      "omnipay/tests": "^4",
       "squizlabs/php_codesniffer": "^3",
       "phpro/grumphp": "^1",
       "php-coveralls/php-coveralls": "^2"