CI: Update travis matrix to test on curl-client v2.

Author: shulard

.travis.yml

@@ -49,7 +49,7 @@ matrix:
 
           # Test with httplug 2.x clients
         - php: 7.2
-          env: DEPENDENCIES="php-http/guzzle6-adapter:^2.0.1"
+          env: DEPENDENCIES="php-http/guzzle6-adapter:^2.0.1 php-http/curl-client:^2.0.0"
 
           # Latest commit to master
         - php: 7.3