Travis CI: Test less combinations:

Only one instance and PHP 5.5 and Symfony 2.7.*
Only 2 hhvm instances

.travis.yml

@@ -10,17 +10,23 @@ cache:
     - $HOME/.composer/cache
 
 php:
-  - 5.5
   - 5.6
   - 7.0
-  - hhvm
 
 env:
-  - SYMFONY_VERSION="2.7.*"
   - SYMFONY_VERSION="2.8.*"
   - SYMFONY_VERSION="3.0.*"
   - SYMFONY_VERSION="3.1.*"
 
+matrix:
+  include:
+    - php: 5.5
+      env: SYMFONY_VERSION="2.7.*"
+    - php: hhvm
+      env: SYMFONY_VERSION="2.8.*"
+    - php: hhvm
+      env: SYMFONY_VERSION="3.0.*"
+
 # Only send code coverage to Code Climate for the current versions of PHP and Symfony LTS
 # https://github.com/doctrine/doctrine2/blob/3570f4a49afc7e98fed71e0596dded6a39d4fd7b/.travis.yml#L16
 before_install: