Method configure() in FileBasedConfigurationBuilder is now overridden.

The return type has been changed to FileBasedConfigurationBuilder. This
simplifies the creation of builder objects because now method chaining is
possible retaining the return type.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1593831 13f79535-47bb-0310-9956-ffa450edef68