Update travis config to test againt 7.0, 7.1, and nightly. Allow nightly failures.

Author: siwinski

.travis.yml

@@ -5,13 +5,19 @@ php:
   - 5.4
   - 5.5
   - 5.6
-  - 7
+  - 7.0
+  - 7.1
+  - nightly
   - hhvm
 
+matrix:
+  allow_failures:
+    - php: nightly
+
 sudo: false
 
 install:
   - composer install --no-interaction
-  
+
 script:
   - vendor/bin/phpunit --coverage-text