Raise minimum PHP version.

.travis.yml

 sudo: false
 language: php
 php:
-  - 5.6
   - 7.0
   - 7.1
+  - 7.2
 
 env:
   - VUFIND_HOME=$PWD VUFIND_LOCAL_DIR=$PWD/local

composer.json

@@ -10,7 +10,7 @@
     "license": "GPL-2.0",
     "config": {
         "platform": {
-            "php": "5.6.0"
+            "php": "7.0.0"
         }
     },
     "require": {