mkimage-debootstrap: set ubuntuLatestLTS as Trusty

Ubuntu 14.04 LTS (Trusty Tahr) was released on Thu, 17 Apr 2014; Update
ubuntuLatestLTS accordingly.

Docker-DCO-1.1-Signed-off-by: Jonathan McCrohan <jmccrohan@gmail.com> (github: jmccrohan)

contrib/mkimage-debootstrap.sh

@@ -43,7 +43,7 @@ usage() {
 debianStable=wheezy
 debianUnstable=sid
 # this should match the name found at http://releases.ubuntu.com/
-ubuntuLatestLTS=precise
+ubuntuLatestLTS=trusty
 # this should match the name found at http://releases.tanglu.org/
 tangluLatest=aequorea