[CI] Updated Travis config

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

* Take advantage of container-based infrastructure
* Added matrix for PHP-5.6, PHP-nightly and HHVM

Author: phansys

.travis.yml

@@ -1,11 +1,23 @@
 language: php
 
-php:
-  - 5.3
-  - 5.4
-  - 5.5
+sudo: false
 
-before_script:
-  - composer --prefer-source --dev install
+before_install:
+    - composer self-update
+
+install:
+    - composer install --prefer-source
+
+matrix:
+    include:
+        - php: 5.3
+        - php: 5.4
+        - php: 5.5
+        - php: 5.6
+        - php: nightly
+        - php: hhvm
+    allow_failures:
+        - php: nightly
+        - php: hhvm
 
 script: ./vendor/bin/phpunit