Do not test on PHP 5.6

RoSk0 · RoSk0 · commit 7d323069775a · 2021-01-12T10:02:01.000+13:00
While still supported we can't test on this PHP version to become PHPUnit 9
compatible.
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,5 @@
 language: php
 php:
-   - '5.6'
    - '7.0'
    - '7.1'
    - '7.2'

-Original file line number
+Diff line change
@@ @@ -1,6 +1,5 @@ @@
 language: php
 php:
 -   - '5.6'
    - '7.0'
    - '7.1'
    - '7.2'