Drop PHP 5.4, 5.5 support

Author: trejjam

.travis.yml

@@ -1,8 +1,6 @@
 language: php
 
 php:
-  - 5.4
-  - 5.5
   - 5.6
   - 7.0
   - 7.1

composer.json

@@ -14,7 +14,7 @@
         "psr-4" : { "Clue\\React\\Shell\\": "src/" }
     },
     "require": {
-        "php": ">=5.4",
+        "php": ">=5.6",
         "react/promise": "~1.0|~2.0",
         "react/child-process": "~0.3.0|~0.4.0|^0.5",
         "react/stream": "~0.3.0|~0.4.0|^0.5|^0.6|^0.7",