Testing on PHP 5.6, reduced variations

Testing every release line against every DB driver and every PHP version tends to increase the complexity of maintaining the builds (more chance of a false negative). It also strains the free resources provided by Travis CI

.travis.yml

@@ -1,17 +1,17 @@
 language: php
-php:
- - 5.3
 
 sudo: false
 
-env:
- - DB=MYSQL CORE_RELEASE=3.1
- - DB=MYSQL CORE_RELEASE=3.2
- - DB=PGSQL CORE_RELEASE=3.1
- - DB=PGSQL CORE_RELEASE=3.2
- - DB=SQLITE3 CORE_RELEASE=3.1
- - DB=SQLITE3 CORE_RELEASE=3.2
- - DB=MYSQL CORE_RELEASE=3.3
+matrix:
+  include:
+    - php: 5.3
+      env: DB=PGSQL CORE_RELEASE=3.1
+    - php: 5.6
+      env: DB=MYSQL CORE_RELEASE=3.2
+    - php: 5.6
+      env: DB=SQLITE3 CORE_RELEASE=3.2
+    - php: 5.6
+      env: DB=MYSQL CORE_RELEASE=3.3
 
 before_script:
  - phpenv rehash