update travis config

Author: Joel Butcher

.travis.yml

@@ -10,12 +10,16 @@ cache:
 
 matrix:
   include:
-    - php: 7.1
-    - php: 7.1
+    - php: 7.3
+    - php: 7.3
       env:
         - deps=low
-    - php: 7.2
-    - php: 7.2
+    - php: 7.4
+    - php: 7.4
+      env:
+        - deps=low
+    - php: 8.0
+    - php: 8.0
       env:
         - deps=low
     - php: nightly