Allow client to enqueue a task with unique option

Changes:

- Added Unique option for clients
- Require go v.13 or above (to use new errors wrapping functions)
- Fixed adding queue key to all-queues set (asynq:queues) when scheduling.

.travis.yml

@@ -2,9 +2,7 @@ language: go
 go_import_path: github.com/hibiken/asynq
 git:
   depth: 1
-env:
-  - GO111MODULE=on # go modules are the default
-go: [1.12.x, 1.13.x, 1.14.x]
+go: [1.13.x, 1.14.x]
 script:
   - go test -race -v -coverprofile=coverage.txt -covermode=atomic ./...
 services: