Add PHP 5.6 and no longer allow failures for 5.5 builds
Add driver 1.4.5 and 1.5.1 to test matrix

Ensure the build prompt for 1.5.x is answered, similar to what was done in doctrine/mongodb-odm@4fe7d418353ba91cce02401ecc7ef72dece27ed9
Merge pull request #176 from doctrine/1.0.x-travis

Add new PHP and driver versions to Travis CI