Gracefully omit the meta file from schema fragments

If the meta.json file is in the same directory as schema fragments,
it can currently get double-combined on `prmd combine` once from the
--meta option and once from being in the given directory. As a
workaround users need to move meta.json out of the schema folder. This
change removes the need to use that workaround if users want the file
in the same directory.
Merge pull request #80 from heroku/gracefull-skip-meta

Gracefully omit the meta file from schema fragments