Remove PHP 5.3, 5.4 and 5.5

For incompatibility with phpunit 5 we remove php version under 5.5

Author: khrizt

.travis.yml

@@ -1,14 +1,12 @@
 language: php
 
 php:
-  - 5.3
-  - 5.4
-  - 5.5
   - 5.6
+  - 7.0
 
 before_script:
   - composer install --prefer-source --no-interaction
 
 script:
   - vendor/bin/phpunit --verbose --coverage-text
-  - vendor/bin/phpcs --standard=PSR2 --extensions=php src tests
+  - vendor/bin/phpcs --standard=PSR2 --extensions=php src tests