update min go version to one compatible with min available libp2p

.travis.yml

@@ -4,7 +4,7 @@ os:
 language: go
 
 go:
-  - 1.11.x
+  - 1.13.x
 
 env:
   global:

go.mod

 module github.com/ipfs/go-ipfs-provider
 
-go 1.12
+go 1.13
 
 require (
 	github.com/cenkalti/backoff v2.2.1+incompatible