Update Go version to 1.13

Author: Vighneswar Rao Bojja

.travis.yml

@@ -3,7 +3,7 @@ sudo: required
 services:
 - docker
 go:
-- "1.12"
+- "1.13"
 script:
 - echo "Nginx ASG sync - commit:${TRAVIS_COMMIT}"
 - make BUILD_IN_CONTAINER=0 all

Makefile

@@ -1,5 +1,5 @@
 GO_DOCKER_RUN = docker run --rm -v $(shell pwd):/go/src/github.com/nginxinc/nginx-asg-sync -v $(shell pwd)/build_output:/build_output -w /go/src/github.com/nginxinc/nginx-asg-sync/cmd/sync
-GOLANG_CONTAINER = golang:1.12
+GOLANG_CONTAINER = golang:1.13
 BUILD_IN_CONTAINER = 1
 
 all: amazon centos7 ubuntu-xenial amazon2 ubuntu-bionic