Use get_config_path to get the path for the predicate_mappings.yml

This simplifies the code for getting this file by leveraging code used
to find the solr.yml and fedora.yml files.  Additionally this fixes a
bug where predicate_mappings.yml is not found in the local rails
application.
Merge pull request #168 from projecthydra/detect_predicate_config

Use get_config_path to get the path for the predicate_mappings.yml