Drop PHP5 support and remove from Travis

This is a personal project, and I belive that php 7 is now a
reasonable requirement.

.travis.yml

@@ -1,7 +1,6 @@
 language: php
 
 php:
-  - '5.6'
   - '7.0'
   - '7.1'
   - '7.2'

composer.json

@@ -41,6 +41,7 @@
     },
     "minimum-stability": "dev",
     "require": {
+        "php": ">=7.0",
         "indeyets/pake": "~1.99",
         "kriswallsmith/assetic": "1.1.2",
         "leafo/lessphp": "v0.4.0",
@@ -62,4 +63,4 @@
         "phpunit/phpunit": "^6.2",
         "squizlabs/php_codesniffer": "^3.0@dev"
     }
-}
+}