DTACLOUD-437 update gemspec for latest rbovirt rubygem 0.0.17

TrackedAt: http://tracker.deltacloud.org/patch/1ea1a5495bb4c795274bf127c4a22eb0f3cd38b1
DTACLOUD-457 RHEVm, Passing an invalid instance ID to start instance results in traceback with 500.

TrackedAt: http://tracker.deltacloud.org/patch/0f9ec84ff0ad613e28d1b49c3038a75010d56393
Previously, if you assigned nil to properties, ent_properties would end up
being "null" which is not valid JSON.