make weave build & work with go 1.5

[rade]

See #1386 for a failed attempt.

[tomwilkie]

Relevant: weaveworks/scope#488

[rade]

ah! "The old format still works in 1.5, just spits a warning.". I'd be happy with that, at least for the time being. so it seems we could just resurrectt #1386 minus that change.

[rade]

so it seems we could just resurrectt #1386 minus that change.

Actually @dpw only reverted the Makefile change, so there is even less to do here (if anything?)

[dpw]

I've reinstated the other Makefile changes that weren't responsible for the problems with 1.4: a67b9f5

So now we have all of #1386 back, except for the -X change, which just means we get a warning with go-1.5.