Update the minimal requirement to php 7.0.

Author: richardregeer

.travis.yml

@@ -1,9 +1,8 @@
 sudo: false
 language: php
 php:
+  - "7.3"
   - "7.2"
   - "7.1"
   - "7.0"
-  - "5.6"
-  - "5.5"
 script: ./test/run

composer.json

@@ -17,7 +17,7 @@
     }
   ],
   "require": {
-    "php": ">=5.5.0"
+    "php": ">=7.0.0"
   },
   "bin": ["bin/coverage-check"]
 }