update travis config for Laravel 6 requirements

Author: michaeldyrynda

.travis.yml

@@ -6,42 +6,10 @@ cache:
 
 matrix:
   include:
-    - php: 5.6.4
-      env: ILLUMINATE_VERSION=5.4.*
-    - php: 7.0
-      env: ILLUMINATE_VERSION=5.4.*
-    - php: 7.0
-      env: ILLUMINATE_VERSION=5.5.*
-    - php: 7.1
-      env: ILLUMINATE_VERSION=5.4.*
-    - php: 7.1
-      env: ILLUMINATE_VERSION=5.5.*
-    - php: 7.1
-      env: ILLUMINATE_VERSION=5.6.*
-    - php: 7.1
-      env: ILLUMINATE_VERSION=5.7.*
-    - php: 7.1
-      env: ILLUMINATE_VERSION=5.8.*
     - php: 7.2
-      env: ILLUMINATE_VERSION=5.4.*
-    - php: 7.2
-      env: ILLUMINATE_VERSION=5.5.*
-    - php: 7.2
-      env: ILLUMINATE_VERSION=5.6.*
-    - php: 7.2
-      env: ILLUMINATE_VERSION=5.7.*
-    - php: 7.2
-      env: ILLUMINATE_VERSION=5.8.*
-    - php: 7.3
-      env: ILLUMINATE_VERSION=5.4.*
-    - php: 7.3
-      env: ILLUMINATE_VERSION=5.5.*
-    - php: 7.3
-      env: ILLUMINATE_VERSION=5.6.*
-    - php: 7.3
-      env: ILLUMINATE_VERSION=5.7.*
+      env: ILLUMINATE_VERSION=6.0.*
     - php: 7.3
-      env: ILLUMINATE_VERSION=5.8.*
+      env: ILLUMINATE_VERSION=6.0.*
 
 before_install: travis_retry composer require "illuminate/database:${ILLUMINATE_VERSION}" "illuminate/events:${ILLUMINATE_VERSION}" --no-update -v