Update minimum PHP version to 7.1 in Travis

.travis.yml

@@ -14,11 +14,11 @@ env:
 matrix:
   fast_finish: true
   include:
-    - php: 5.6
-      env: DB=MYSQL PHPUNIT_TEST=1 PHPCS_TEST=1
-    - php: 7.0
-      env: DB=MYSQL PHPUNIT_TEST=1
     - php: 7.1
+      env: DB=MYSQL PHPUNIT_TEST=1 PHPCS_TEST=1
+    - php: 7.2
+      env: DB=MYSQL PHPUNIT_TEST=1
+    - php: 7.3
       env: DB=MYSQL PDO=1 PHPUNIT_COVERAGE_TEST=1
 
 before_script: