Use Github generated PR message for merge commits

[pratikmallya]

• pull in latest version of go-github
• fix unit tests

Fixes: #845

[pratikmallya]

@lkysow a bit unsure about how I would update to use the latest version of go-github.... the naive go get seems to fail with:

$ go get -u github.com/google/go-github@master
go: finding github.com/google/go-github master
go: finding github.com/google/go-github master
go get github.com/google/go-github@master: github.com/google/go-github@v0.0.0-20200302001034-5f66efb9dc28: invalid version: go.mod has post-v0 module path "github.com/google/go-github/v29" at revision 5f66efb9dc28

[lkysow]

Do we need to use import github.com/google/go-github/v29/github?

[lkysow]

Closing in favour of #1049

[pratikmallya]

Thanks @lkysow ! Apologies, got distracted with other things and totally dropped the ball here.

[lkysow]

No worries! I didn't test it out myself so hopefully it works!