Fixing an issue occuring with php 5.3

Implementing interfaces methods as abstracts in an abstract class doesn't work with PHP 5.3 (it does with 5.4 however). Removing those methods abstract implementations fixes this.
Merge pull request #68 from Bladrak/patch-1

Fixing an issue occuring with php 5.3
Added PHP 5.5 to .travis.yml
Merge pull request #69 from pulzarraider/patch-1

Added PHP 5.5 to .travis.yml
Merge branch 'sonata/master'