disable 7.1 testing completely

currently phpseclib throws an error (about a deprecated function).
errors do not count as failures, so we can't skip them. so for now
7.1 testing is removed again.

.travis.yml

 language: php
 sudo: false
 php:
-  - "7.1"
   - "7.0"
   - "5.6"
   - "5.5"
@@ -15,7 +14,6 @@ matrix:
   allow_failures:
     - php:
         - "hhvm"
-        - "7.1"
 notifications:
   irc:
     channels: