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: Ia5aad1cc2f6594fb6d7ec92cb0de32e5bfebc82b
Signed-off-by: Ryan Barry <rbarry@redhat.com>