Test against 3.3 core and PHP 5.6 (#93)

* Removed builds for unsupported 3.0 release line

* Added 3.3 release testing

Note that we're trying to make this smarter going forward, just testing against `3@stable` (see https://github.com/silverstripe-labs/silverstripe-travis-support/issues/33)

* 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

@@ -1,19 +1,17 @@
 language: php
-php:
- - 5.3
 
 sudo: false
 
-env:
- - DB=MYSQL CORE_RELEASE=3.0
- - DB=MYSQL CORE_RELEASE=3.1
- - DB=MYSQL CORE_RELEASE=3.2
- - DB=PGSQL CORE_RELEASE=3.0
- - DB=PGSQL CORE_RELEASE=3.1
- - DB=PGSQL CORE_RELEASE=3.2
- - DB=SQLITE3 CORE_RELEASE=3.0
- - DB=SQLITE3 CORE_RELEASE=3.1
- - DB=SQLITE3 CORE_RELEASE=3.2
+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