Access keyserver.ubuntu.com on port 80

[Sauraus]

This resolves #264

[Sauraus]

Those Travis errors already exist on trunk, which forked a few minutes before submitting my pull request.

[tduffield]

So I'm not super convinced that port 80 should be the default. I think it would be better to make a note in the README about this and to offer an example about how you would override this in your local environment.

[djdefi]

I agree there @tduffield

[gionn]

@tduffield @rtrauntvein state reasons instead of opinions, this is a good workaround for users behind restrictive firewalls and should cause no-harm to everyone else.

[tduffield]

@Sauraus if you rebase this PR on master and Travis passes I'll merge it in. Given that Docker itself uses port 80 in it's installation procedures I no longer see a reason to enforce the default.

[Sauraus]

@tduffield rebased and recommitted.

NOTE: The travis failure was already present on master, not my doing. ;)