Dropped install and added 5.6 as an allowed failure

Still no dice on HHVM even with installing php5-dev. Added PHP 5.6 to see how
we fare. Will drop the allowed failure when 5.6 is gold or when the test suite
is passing. Whichever comes first.

.travis.yml

@@ -3,10 +3,12 @@ language: php
 php:
   - 5.4
   - 5.5
+  - 5.6
   - hhvm
 
 matrix:
   allow_failures:
+    - php: 5.6
     - php: hhvm
 
 services:
@@ -14,10 +16,6 @@ services:
   - memcached
   - redis
 
-before_install:
-  - sudo apt-get update
-  - sudo apt-get install php5-dev
-
 install:
   - composer install
   - mkdir vendor/php-test-helpers