Update the .travis.yml file

Build against the legacy 1.9.7 and 1.13 go versions. 1.9.7 is the oldest
go version that supports modules (and the first in 1.9.x release that
does it). `go.mod` specifies the latter version.

Author: nightuser

.travis.yml

@@ -1,15 +1,17 @@
 language: go
-sudo: false
 go_import_path: gopkg.in/go-oauth2/redis.v3
-go:
-  - 1.9
+matrix:
+  include:
+    - go: "1.9.7"
+    - go: 1.13.x
+      env: GO111MODULE=on
 services:
-  - redis-server
+  - redis
 before_install:
   - go get -t -v ./...
 
 script:
   - go test -race -coverprofile=coverage.txt -covermode=atomic
 
 after_success:
-  - bash <(curl -s https://codecov.io/bash)
+  - bash <(curl -s https://codecov.io/bash)