Bump PHP versions on Travis

These are relevant now.

Also had to explicitly set xdebug mode on versions using XDebug 3.

Author: jtojnar

.travis.yml

@@ -2,6 +2,18 @@ language: php
 
 php:
   - 5.6
+  - 7.0
+  - 7.1
+  - 7.2
+  - 7.3
+    env:
+      - XDEBUG_MODE=coverage
+  - 7.4
+    env:
+      - XDEBUG_MODE=coverage
+  - 8.0
+    env:
+      - XDEBUG_MODE=coverage
 
 before_script:
   - composer install --dev