portability: unf_ext gem version available that works on ARM

[vielmetti]

(Part of #802 , pulled out separately)

I'm working to port the code to an ARM system, and ran into this issue.

The referenced issue knu/ruby-unf_ext#26 notes that the unf_ext gem fails to build in its rubygems version on arm64 and ppc64le processors, but does build from source on github due to a few extra patches applied since the gem was updated. There's a request upstream to update the gem.

Three ways forward:

• fix upstream by releasing a new version of the gem with the fix included
• work around in Dockerfile.{armhf,aarch64} by installing from source, not from the gem
• update the Gemfile with gem 'uef_ext', git: 'https://github.com/knu/ruby-unf_ext.git' and a specific branch

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• I searched or browsed the repo’s other issues to ensure this is not a duplicate.

[vielmetti]

Here's the patch that fixes Diaspora:

https://gist.github.com/mlamberti/90bd723ef41ac0a5862c80dd38b76db7