Fix issue with rubygems 3.0.0 by pinning it to 2.7.8

Description

Rubygems 3.0.0 was released recently and it is causing the following error when provisioning the forum service:

```
/edx/app/forum/.rbenv/versions/2.4.1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file – rubygems/core_ext/kernel_warn (LoadError)
from /edx/app/forum/.rbenv/versions/2.4.1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from /edx/app/forum/.rbenv/versions/2.4.1/lib/ruby/site_ruby/2.4.0/rubygems.rb:1395:in `<top (required)>'
from <internal:gem_prelude>:4:in `require'
from <internal:gem_prelude>:4:in `<internal:gem_prelude>'}}
So we pin it to 2.7.8.
```

Already reviewed in https://github.com/open-craft/configuration/pull/66.

Assignee

Unassigned

Reporter

Open Source Pull Request Bot

Contributor Name

Usman Khalid

Repo

edx/configuration

Customer

Epic Link

None

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None

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Priority

Unset
Configure