updating tested PHP versions in travis CI configuration

.travis.yml

@@ -1,9 +1,11 @@
 language: php
 sudo: false
 php:
-  - 5.5
-  - 5.6
-  - 7.0
+  - '5.4'
+  - '5.5'
+  - '5.6'
+  - '7.0'
+  - '7.1'
 
 install:
   - rm -rf ~/.nvm && git clone https://github.com/creationix/nvm.git ~/.nvm && (cd ~/.nvm && git checkout `git describe --abbrev=0 --tags`) && source ~/.nvm/nvm.sh && nvm install 4