MINOR: Refine build matrix.

It's important to test different databases. It's important to test
different versions of PHP. But we don't need to test all possible
versions of each tested in a cartesian product.

This patch ensures that there are 4 builds that cover:

 - PHP 5.4, 5.5, 5.6
 - MySQL, PostgreSQL and SQLite
 - PDO and each db-specific connector
 - Behat and non

.travis.yml

@@ -7,9 +7,6 @@ addons:
     packages:
       - tidy
 
-php:
-  - 5.4
-
 env:
   global:
     - CORE_RELEASE=master
@@ -17,20 +14,16 @@ env:
     - "ARTIFACTS_S3_BUCKET=silverstripe-travis-artifacts"
     - secure: "DjwZKhY/c0wXppGmd8oEMiTV0ayfOXiCmi9Lg1aXoSXNnj+sjLmhYwhUWjehjR6IX0MRtzJG6v7V5Y+4nSGe+i+XIrBQnhPQ95Jrkm1gKofX2mznWTl9npQElNS1DXi58NLPbiB3qxHWGFBRAWmRQrsAouyZabkPnChnSa9ldOg="
     - secure: "UmbXCNLK0f2Dk+7qX8bOVcgIt4QhRvccoWvMUxaPtIU+95HCbG10eeCxvfOeBax+tHcRXmeCG4vM4tcuT/WoANkAma/VX74DylFjbWhks2tsKOcr2kjTrOwe6Q9CXOBjVAlcx0lnV/a+w83KARjXGnCrIbE7p7r4EDw31rkVufg="
-  matrix:
-    - DB=MYSQL
-    - DB=SQLITE
-    - DB=PGSQL
 
 matrix:
   include:
     - php: 5.4
-      env: DB=MYSQL PDO=1
+      env: DB=PGSQL
     - php: 5.5
-      env: DB=MYSQL
+      env: DB=SQLITE
+    - php: 5.6
+      env: DB=MYSQL PDO=1
     - php: 5.6
-      env: DB=MYSQL
-    - php: 5.4
       env: DB=MYSQL BEHAT_TEST=1
 
 before_script: