Bump Go versions for Travis, fix tests

Replace gometalinter with golangci-lint.

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

Fix linter warnings.

api/publish.go

@@ -300,7 +300,7 @@ func apiPublishUpdateSwitch(c *gin.Context) {
 			}
 
 			snapshot, err2 := snapshotCollection.ByName(snapshotInfo.Name)
-			if err != nil {
+			if err2 != nil {
 				c.AbortWithError(404, err2)
 				return
 			}