Travis: Start testing on PHP 7.0 (nightly).

This add testing on PHP 7.0 (nightly build). The test is added as an
allowed failure for now. but it can be useful to keep an eye on it just
in case we do something that won't work in PHP 7.

.travis.yml

@@ -4,10 +4,12 @@ php:
 - 5.4
 - 5.5
 - 5.6
+- 7.0
 - hhvm
 matrix:
   allow_failures:
   - php: hhvm
+  - php: 7.0
 before_script: composer install --dev
 script: phpunit --configuration tools/phpunit
 after_script: