Revert default Ubuntu release back to Saucy

Trusty ramdisks do not boot, failing to exec init.  This reverts
the default DIB_RELEASE back to Saucy until Trusty ramdisks have
been properly tested.

Change-Id: I5ac81822b78805275268cecc021a74a62c22ef58
Merge "Revert default Ubuntu release back to Saucy"