Added nightly building to Travis

Let's us check PHP 7 compatibility

.travis.yml

@@ -4,8 +4,13 @@ php:
   - 5.4
   - 5.5
   - 5.6
+  - nightly
   - hhvm
 
+matrix:
+  allow_failures:
+    - php: nightly
+
 sudo: false
 
 install: travis_retry composer install --no-interaction --ignore-platform-reqs --no-scripts --prefer-source