Drop php7.1 support

Author: christiaan

.travis.yml

@@ -5,15 +5,8 @@ branches:
 language: php
 
 php:
-  - '7.1'
   - '7.2'
 
-matrix:
-  include:
-    - name: Test with the lowest allowed versions of dependencies
-      php: '7.1'
-      install: composer update --prefer-lowest
-
 cache:
   directories:
     - $HOME/.composer/cache