caddy: build only with go 1.6

[davidak]

Things done

• Tested using sandboxing (nix-build --option build-use-chroot true or nix.useChroot on NixOS)
• Built on platform(s)
  • NixOS
  • OS X (fails because of broken stdenv stdenv-darwin build problem #14704)
  • Linux (tested on debian 7)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip" (nox explodes (git can't checkout object madjar/nox#43), but should be fine. no pkgs depend on this)
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

caddys readme suggests:

Note: You will need Go 1.6 or newer.

@angus-g @joachifm

[davidak]

👍 ready to merge

[angus-g]

Looks good to me. Just wondering if it should be disabled = isGo14 || isGo15 to be in-line with the way terraform does it. (Probably a minor point)

[davidak]

@angus-g net_go14 does it this way. maybe we don't need that line at all because the version is set by go16Packages, but it's more safe that it newer get's build with an incompatible version. your suggested way would support go1.7 and up, so i changed it. thanks!