Bump Go version to 1.13 for Travis (#42)

.travis.yml

@@ -16,6 +16,7 @@ go:
   - 1.10.x
   - 1.11.x
   - 1.12.x
+  - 1.13.x
   - tip
 
 matrix:
@@ -35,11 +36,11 @@ matrix:
       go: 1.6.x
 
 before_install:
-  - if [[ $TRAVIS_GO_VERSION == 1.12* ]]; then go get github.com/axw/gocov/gocov github.com/mattn/goveralls; fi
+  - if [[ $TRAVIS_GO_VERSION == 1.13* ]]; then go get github.com/axw/gocov/gocov github.com/mattn/goveralls; fi
 
 script:
   - go test -v -tags ci ./...
-  - if [[ $TRAVIS_GO_VERSION == 1.12* ]]; then $GOPATH/bin/goveralls -service=travis-ci -ignore="examples/callbacks/callbacks.go,examples/dataloader/dataloader.go,examples/mycachedapp/mycachedapp.go"; fi
+  - if [[ $TRAVIS_GO_VERSION == 1.13* ]]; then $GOPATH/bin/goveralls -service=travis-ci -ignore="examples/callbacks/callbacks.go,examples/dataloader/dataloader.go,examples/mycachedapp/mycachedapp.go"; fi
 
 notifications:
   email: