Update Go version to 1.13

Vighneswar Rao Bojja · bvighnesha · commit 56b5d6b19c14 · 2019-10-04T14:51:03.000+05:30
diff --git a/.travis.yml b/.travis.yml
@@ -3,7 +3,7 @@ sudo: required
 services:
 - docker
 go:
-- "1.12"
+- "1.13"
 script:
 - echo "Nginx Prometheus Exporter - commit:${TRAVIS_COMMIT}"
 - make test
diff --git a/go.mod b/go.mod
@@ -1,6 +1,6 @@
 module github.com/nginxinc/nginx-prometheus-exporter
 
-go 1.12
+go 1.13
 
 require (
 	github.com/nginxinc/nginx-plus-go-client v0.5.0
