Add sponsors to notification emails

[svenfuchs]

We display 1 Platinum sponsor or 2 Gold sponsors below every build notification email. These are randomized and Platinum sponsors have a 2:1 chance to appear over Gold sponsors. There is a link to the main Travis CI sponsors page 3

[svenfuchs]

@rkh can you take care of this? can't assign you to this ticket right now (GH issue?)

[svenfuchs]

btw we still have the option to only add text links and a short copy to these emails in order to not make them too spammy

afaik mathias favors this, i'd probably opt for it, too

i'll update the sponsor plans page right away, just to be on the safe side

[svenfuchs]

Done.