Revert accidental PHP version update to develop branch

.travis.yml

@@ -2,6 +2,7 @@ language: php
 dist: trusty
 
 php:
+  - 5.6
   - 7.0
   - 7.1
   - 7.2
@@ -25,13 +26,13 @@ jobs:
   include:
 
     - stage: Code style
-      php: 7.3
+      php: 7.2
       script:
         - ./vendor/bin/php-cs-fixer fix --diff --verbose --dry-run
         - ./vendor/bin/phpcs --report-width=200 samples/ src/ tests/ --ignore=samples/Header.php --standard=PSR2 -n
 
     - stage: Coverage
-      php: 7.3
+      php: 7.2
       env: KEEP_XDEBUG=1
       script:
         - travis_wait 40 ./vendor/bin/phpunit --debug --coverage-clover coverage-clover.xml
@@ -40,7 +41,7 @@ jobs:
         - php ocular.phar code-coverage:upload --format=php-clover tests/coverage-clover.xml
 
     - stage: API documentation
-      php: 7.3
+      php: 7.2
       before_script:
       - curl -O https://get.sensiolabs.org/sami.phar
       script:

composer.json

@@ -39,7 +39,7 @@
         ]
     },
     "require": {
-        "php": "^7.0",
+        "php": "^5.6|^7.0",
         "ext-ctype": "*",
         "ext-dom": "*",
         "ext-gd": "*",