(#4135) Update pluginsync to only load ruby files.

Pluginsync allows replication of all files, but we should only load
ruby files available in the pluginsync directory. For example,
currently pluginsync will attempt and fail to load puppet-module
generated readme files. This change restrict puppet agent to only load
ruby files, but preserve the ability to sync other files such as
scripts in other languages, augeus lens, etc.