Add PHP7 to automated builds

Just read that Travis has added support for "nightly" PHP7.
diff --git a/.travis.yml b/.travis.yml
index 4f56044..26b194f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,6 +6,7 @@
   - 5.4
   - 5.5
   - 5.6
+  - 7
   - hhvm
   
 env:
@@ -31,11 +32,14 @@
   allow_failures:
     - php: 5.2
     - php: hhvm
+    - php: 7
   exclude:
     - php: hhvm
       env: DB=pgsql
     - php: hhvm
       env: DB=pdo/pgsql
+    - php: 7
+      env: mysql
     - php: 5.2
       env: DB=sqlite
     - php: 5.2