Run unit tests on Travis CI with php nightlies

With PHP7 in active development, this will help us identify issues there. Also will be fun to see how fast it is.

Fixes #31454



git-svn-id: https://develop.svn.wordpress.org/trunk@31558 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -16,8 +16,11 @@ matrix:
     env: WP_TRAVISCI=travis:phpunit
   - php: hhvm
     env: WP_TRAVISCI=travis:phpunit
+  - php: nightly
+    env: WP_TRAVISCI=travis:phpunit
   allow_failures:
   - php: hhvm
+  - php: nightly
   fast_finish: true
 before_install:
 - WP_CORE_DIR=/tmp/wordpress/