remove PHP 5.5 from CI matrix

Dev dependencies start dropping support for PHP 5.5 now. Because the
5.2 to 5.4 are already missing in the CI matrix, we drop explicit 5.5
builds for now and rely on static compatibility checks.

.travis.yml

@@ -1,7 +1,5 @@
 language: php
-dist: trusty
 php:
-- '5.5'
 - '5.6'
 - '7.0'
 - '7.1'