Pin ffi gem version in Gemfile

[adfoster-r7]

Description

Fix CI build failure when running in Ruby 2.7.5 docker environment:

docker run -it -w $(pwd) -v $(pwd):$(pwd) --rm ruby:2.7.5 /bin/sh

Before:

# bundle
Fetching gem metadata from https://rubygems.org/.........
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
ffi-1.17.0-x86_64-linux-musl requires rubygems version >= 3.3.22, which is incompatible with the current version, 3.1.6

After:

# bundle
...
Bundle complete! 12 Gemfile dependencies, 49 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
# 

Motivation and Context

Fix CI failing on ffi dependency issues

How Has This Been Tested?

CI passes

Types of changes

• Bug fix

Checklist:

• I have updated the documentation accordingly (or changes are not required).
• I have added tests to cover my changes (or new tests are not required).
• All new and existing tests passed.