Persist the first version in a pref and include it in omaha req attrs

The pref is named "aleph-version", and the omaha req app attr
"alephversion":
$ cat /var/lib/update_engine/prefs/aleph-version
444.0.0+2014-09-22-1720

$ journalctl -u update-engine | grep aleph
Sep 25 00:08:20 coreos_developer_qemu-444-0-0-20 update_engine[1791]:
<app appid="{e96281a6-d1af-4bde-9a0a-97b76e56dc57}"
version="444.0.0+2014-09-22-1720" track="developer"
bootid="{60f9cf6a-361a-45f1-bb86-22380189907a}" oem="" oemversion=""
alephversion="444.0.0+2014-09-22-1720"
machineid="83ab5976451549a1afede31db82fd215" lang="en-US"
board="amd64-usr" hardware_class="" delta_okay="false" >
Merge pull request #49 from vcaputo/alephversion

Persist the first version in a pref and include it in omaha req attrs