Bump Go versions for Travis, fix tests

Replace gometalinter with golangci-lint.

Fix system tests (wheezy is gone, replace with stretch).

Fix linter warnings.

system/t06_publish/PublishSnapshot35Test_release

@@ -1,7 +1,7 @@
 Origin: Debian
-Label: . wheezy
-Suite: wheezy
-Codename: wheezy
+Label: . stretch
+Suite: stretch
+Codename: stretch
 Architectures: amd64 i386
 Components: main
 Description: Generated by aptly