Juju should be built with go 1.6

[Brad Crittenden]

> On Aug 22, 2016, at 13:11 , Nate Finch <nate.finch at canonical.com> wrote:
> 
> 
> // This line intentionally does not compile.  This file will only be compiled if
> // you are compiling with a version of Go that is lower than the one we support.
> var requiredGoVersion = This_project_requires_Go_1_6
> 
> 
> If this file is compiled on a version of go that is 1.6 or higher, it'll get excluded from the build, and all will be well. If you compile it on a version of go that is lower than 1.6, you'll get an error like this:
> ./version_canary.go:10: undefined: This_project_requires_Go_1_6
> This is (hopefully) a much clearer error message than something like this:
> ../utils/tls.go:24: undefined: tls.TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
> (which is the error message we used to get if we compiled with an earlier version of go).

Thanks for this Nate. I got caught by this last week when setting up a CI machine and it took me a while to figure out.

—Brad