Remove PHP 7.1 from travis.yml

Guzzle 7 requires PHP 7.2.5, so we are doing the same here.

Author: jbeales

.travis.yml

@@ -10,7 +10,6 @@ cache:
 
 matrix:
   include:
-    - php: 7.1
     - php: 7.2
     - php: 7.3
     - php: 7.4