Fedora 19 is no longer in development. Bump up to 20 in recipe/Makefile.am

Move CUR_DEVEL up to 20 so we don't pull updates-testing into F19
images.

Change-Id: I4c34914514342a9993598ee89c83be175d811988
Signed-off-by: Ryan Barry <rbarry@redhat.com>