Removed duplicate PHP 5.5 and 5.4 entries from travis.yml

As suggested by @derrabus, PHP 5.5 and 5.4 are also added through the matrix, so they're unnecessary in the php version list.

Co-Authored-By: Alexander M. Turek <me@derrabus.de>

.travis.yml

@@ -13,7 +13,7 @@ env:
         - SYMFONY_PHPUNIT_REMOVE='' # don't remove prophecy
         - SYMFONY_PHPUNIT_REMOVE_RETURN_TYPEHINT=1
 
-php: [7.2, 7.1, 7.0, 5.6, 5.5, 5.4]
+php: [7.2, 7.1, 7.0, 5.6]
 
 matrix:
     include: