Add hhvm to tests, remove composer update

Author: amacneil

.travis.yml

@@ -4,6 +4,7 @@ php:
   - 5.3
   - 5.4
   - 5.5
+  - hhvm
 
 env:
   - SYMFONY_VERSION="2.1" GUZZLE_VERSION="3.1"
@@ -12,8 +13,6 @@ env:
   - SYMFONY_VERSION="2.*" GUZZLE_VERSION="3.*"
 
 before_script:
-  - composer self-update
-  - composer --version
   - composer require symfony/http-foundation:${SYMFONY_VERSION} --no-update
   - composer require guzzle/http:${GUZZLE_VERSION} --no-update
   - composer install -n --dev --prefer-source