NEW: Ensure php7 builds pass.

SilverStripe 4 should work in php7, so we shouldn't accept its failure.

.travis.yml

@@ -28,6 +28,8 @@ matrix:
       env: DB=MYSQL PDO=1
     - php: 5.6
       env: DB=MYSQL BEHAT_TEST=1 JS_SANITY_CHECK=1
+    - php: 7.0
+      env: DB=MYSQL PDO=1
 
 before_script:
 # Until http://pecl.php.net/package/imagick is working again