Switch to "debian:jessie" as the base

Fixes #11

@proppy @yosifkit time to vote with your LGTMs :wink:

Anyone else: last chance to speak out if you don't want this.  I think the general consensus is that Wheezy is too old for Go, though.  Also, this is slightly less dangerous seeing that Jessie's freeze is in a month from now, so we're not likely to see any really major changes, especially to the few packages we actually rely on in these images.