update Travis configuration

* run builds on the container-based infrastructure
* add a build for PHP 7
* report the build status as soon as possible (either when a mandatory
  job fails or when only jobs allowed to fail are left)

.travis.yml

@@ -1,3 +1,5 @@
+sudo: false
+
 language: php
 
 php:
@@ -5,6 +7,7 @@ php:
   - 5.4
   - 5.5
   - 5.6
+  - 7.0
   - hhvm
 
 env:
@@ -12,6 +15,7 @@ env:
     - deps=no
 
 matrix:
+  fast_finish: true
   include:
     - php: 5.3
       env: deps=low