Laravel 8 compatibility (mghoneimy#36)

Require guzzle 7 as a dependency to support Laravel 8
- Add dependency on guzzle 7.0.1 alongside guzzle 6
- Add PHP7.4 to travis config

Author: ngunyimacharia

.travis.yml

@@ -3,6 +3,7 @@ php:
   - '7.1'
   - '7.2'
   - '7.3'
+  - '7.4'
 
 before_script:
   - composer install
@@ -12,4 +13,4 @@ script: vendor/phpunit/phpunit/phpunit tests/ --whitelist src/ --coverage-clover
 after_script:
   - php vendor/bin/codacycoverage clover build/coverage/xml
 
-os: linux
+os: linux

composer.json

@@ -33,7 +33,7 @@
         "ext-json": "*",
         "psr/http-message": "^1.0",
         "psr/http-client": "^1.0",
-        "guzzlehttp/guzzle": "^6.3"
+        "guzzlehttp/guzzle": "^6.3|^7.0.1"
     },
     "require-dev": {
         "phpunit/phpunit": "^7.5|^8.0",