Merge pull request #314 from jalcine/feature/ignoring-thy-sources

Ignoring Your Sources
Export PS4 when RBENV_DEBUG is set for more informative debug output
Add `rbenv --debug <command>` as a shortcut for setting RBENV_DEBUG=1
Suppress shell warnings when hashing is disabled by `set +h`
Guard against exported `CDPATH` (fixes #316)
Merge branch 'develop'

* develop:
  Guard against exported `CDPATH` (fixes #316)
  Suppress shell warnings when hashing is disabled by `set +h`
  Add `rbenv --debug <command>` as a shortcut for setting RBENV_DEBUG=1
  Export PS4 when RBENV_DEBUG is set for more informative debug output