Bugfix

This is related to issue #216.

PHPFileDefinitionSource source is too specific. We should be able to have ChainableDefinitionSource sources.

If the parameter of addDefinitions method (in this builder) was a ChainableDefinitionSource object and not a PHPFileDefinitionSource object, the build process failed. This commit corrects that.

It now allows to use ChainableDefinitionSource in addDefinitions. For example, we can now load a config array from cache, create the ChainableDefinitionSource and pass it instead of a file name.