Make sure jenkins never gets auto-updated.

Last night, jenkins got auto-updated to the latest version, which
fixed a security hole, which caused it to get added to the
trusty-security repo, which caused our security cronjob to
automatically update it.

Sadly, this broke everything.  Our plugins aren't all ready for the
new version of jenkins yet.

So now I mark jenkins as being un-updateable, to make sure this
doesn't happen again.

Auditors: chris

Test Plan:
Ran
   sudo apt-mark hold jenkins
manually on jenkins, then ran
   aptitude safe-upgrade -s -o Aptitude::Delete-Unused=false --assume-yes --target-release `lsb_release -cs`-security
and saw it suggested no packages to update, unlike before the
'apt-mark' call where it suggested updating jenkins.