Add 'templates' directory to built gem

This contains a file ('twiggrc.yml') that is expected by `twigg init`.

Fixes: https://github.com/causes/twigg/issues/13

Change-Id: I46e35c0e12e684b4c0381d518e0769cce71a59f0
Reviewed-on: https://gerrit.causes.com/27333
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>