Bump PHP version to 5.6 for 3.4.

.travis.yml

@@ -2,7 +2,6 @@ language: php
 
 php:
   - 7.0
-  - 5.5
   - 5.6
   - 7.1
 

composer.json

@@ -18,7 +18,7 @@
         "source": "https://github.com/cakephp/cakephp"
     },
     "require": {
-        "php": ">=5.5.9",
+        "php": ">=5.6.0",
         "ext-intl": "*",
         "ext-mbstring": "*",
         "cakephp/chronos": "~1.0",