Update travis resiliency (#23)

* Update travis resiliency

.travis.yml

@@ -1,11 +1,11 @@
 language: go
 go:
-  - 1.7.x
-  - 1.8.x
-  - 1.9.x
+  - 1.11.x
+  - 1.12.x
+  - 1.13.x
   - tip
 before_install:
-  - go get github.com/mattn/goveralls
+  - travis_retry go get github.com/mattn/goveralls
 script:
-  - go test -v $(go list ./... | grep -v /vendor/)
-  - $HOME/gopath/bin/goveralls -service=travis-ci
+  - travis_retry go test -v ./...
+  - travis_retry $HOME/gopath/bin/goveralls -service=travis-ci