update travis config for Laravel 5.6, PHP 7.1/7.2

Author: michaeldyrynda

.travis.yml

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