Gitpod fix: Bump Erlang's Ubuntu repository

[topher200]

Issues addressed

Error when starting gitpod:

1. Navigate to https://gitpod.io/#https://github.com/bors-ng/bors-ng

2. Observe error:

#5 6.300 Some packages could not be installed. This may mean that you have
#5 6.300 requested an impossible situation or if you are using the unstable
#5 6.300 distribution that some required packages have not yet been created
#5 6.300 or been moved out of Incoming.
#5 6.300 The following information may help to resolve the situation:
#5 6.300 
#5 6.300 The following packages have unmet dependencies:
#5 6.376  esl-erlang : Depends: libwxgtk2.8-0 but it is not installable or
#5 6.376                        libwxgtk3.0-0 but it is not installable or
#5 6.376                        libwxgtk3.0-0v5 but it is not installable

Summary of changes

Update repo to correct Ubuntu version.

Fixes #1570

Test plan

I created a gitpod from this PR and confirmed it booted into Gitpod correctly: https://gitpod.io/#https://github.com/bors-ng/bors-ng/pull/1571

(note that bors doesn't start correctly, but that's a different error. we're getting further than before!)

[topher200]

bors try

[bors]

🔒 Permission denied

Existing reviewers: click here to make topher200 a reviewer

[notriddle]

bors try

[bors]

try

Build succeeded:

• Community-TC (push)
• ci

[notriddle]

bors r+

[bors]

Build succeeded:

• ci
• Community-TC (push)