travis: update to go 1.13

.travis.yml

@@ -1,6 +1,6 @@
 language: go
 go:
-  - 1.11.x
+  - 1.13.x
 sudo: false
 matrix:
   include:
@@ -9,7 +9,6 @@ matrix:
     - go get -u golang.org/x/lint/golint
     - go get -u golang.org/x/tools/cmd/...
     script:
-    - export GO111MODULE=on
     - go mod download
     - go test -v -race $(go list ./...)
     - go vet $(go list ./...)

go.mod

@@ -1,5 +1,7 @@
 module github.com/chihaya/chihaya
 
+go 1.13
+
 require (
 	github.com/SermoDigital/jose v0.0.0-20180104203859-803625baeddc
 	github.com/alicebob/gopher-json v0.0.0-20180125190556-5a6b3ba71ee6 // indirect