squashto: Remove support for PHP <7.3 (but it is 7.2 now!)

Author: mmoll

.travis.yml

@@ -1,6 +1,7 @@
 language: php
 
 php:
+    - 7.2 # security support until 30/11/2020
     - 7.3 # security support until 06/12/2021
     - 7.4 # security support until 28/11/2022
     - 8.0 # security support until 26/11/2023