Align with terraform and go-github Go versions

[benj-fletch]

I have noticed that we are out of sync with the Go versions of 2 main dependencies that this repository has:

• Terraform itself uses go version 1.12 (See here)
• go-github uses go version 1.13 (See here)

Currently we are using Go v1.11 in our repo. (e.g. in the Travis CI build

I propose that we need to push our version of Go to at least v1.13 otherwise we will encounter issues when trying to pull in new features from those libraries.

This was evidenced in #369 where a previous commit had updated Terraform to v0.12.20 which introduced a Go SDK method present in go 1.12 onwards and failed the Travis build until this was rectified.

Happy to hear opinions on this update and any issues that we might see