fix: remove require of RubyInline

[sk-]

For some reason the commit added (0c89d72) to remove RubyInline is not the same as PR #24, and a require was left in the code.

That require fails, as the dependency is no longer specified. And that makes the test to fail:

$ bundle exec rake test_units
/Users/coder/.rbenv/versions/3.3.1/lib/ruby/3.3.0/bundled_gems.rb:74:in `require': cannot load such file -- inline (LoadError)
	from /Users/coder/.rbenv/versions/3.3.1/lib/ruby/3.3.0/bundled_gems.rb:74:in `block (2 levels) in replace_require'
	from /Users/coder/code/xirr/lib/xirr/base.rb:7:in `<module:Base>'
	from /Users/coder/code/xirr/lib/xirr/base.rb:5:in `<module:Xirr>'
	from /Users/coder/code/xirr/lib/xirr/base.rb:3:in `<top (required)>'
	from /Users/coder/.rbenv/versions/3.3.1/lib/ruby/3.3.0/bundled_gems.rb:74:in `require'
	from /Users/coder/.rbenv/versions/3.3.1/lib/ruby/3.3.0/bundled_gems.rb:74:in `block (2 levels) in replace_require'
	from /Users/coder/code/xirr/test/test_helper.rb:12:in `<top (required)>'
	from /Users/coder/code/xirr/test/test_cashflow.rb:1:in `require_relative'
	from /Users/coder/code/xirr/test/test_cashflow.rb:1:in `<top (required)>'
	from /Users/coder/.rbenv/versions/3.3.1/lib/ruby/3.3.0/bundled_gems.rb:74:in `require'
	from /Users/coder/.rbenv/versions/3.3.1/lib/ruby/3.3.0/bundled_gems.rb:74:in `block (2 levels) in replace_require'
	from /Users/coder/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/rake-13.2.1/lib/rake/rake_test_loader.rb:21:in `block in <main>'
	from /Users/coder/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/rake-13.2.1/lib/rake/rake_test_loader.rb:6:in `select'
	from /Users/coder/.rbenv/versions/3.3.1/lib/ruby/gems/3.3.0/gems/rake-13.2.1/lib/rake/rake_test_loader.rb:6:in `<main>'
rake aborted!
Command failed with status (1)
/Users/coder/.rbenv/versions/3.3.1/bin/bundle:25:in `load'
/Users/coder/.rbenv/versions/3.3.1/bin/bundle:25:in `<main>'
Tasks: TOP => test_units
(See full trace by running task with --trace)

After the change, the tests are working again.

PS: in a later PR I will be configuring Github Actions for this repo.

[sk-]

@tubedude is there any chance this PR could get reviewed and merged. I'm planning on sending more Prs, but would need confirmation this project is still alive, and that you are willing to receive external contributions.