api: Bump version to 5.

version 5 adds support for fallocate & discard.

Change-Id: Ifdc226953a25e4a4eb63e55d2a7c7ec77c397215
BUG: 953694
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/5276
Reviewed-by: Bharata B Rao <bharata.rao@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>