Add observer gem as a dependency

[AaronRustad]

There is a warning when starting up retest that the observer gem will be removed in Ruby 3.4. This PR adds the dependency in.

/Users/xxx/.asdf/installs/ruby/3.3.0/lib/ruby/gems/3.3.0/gems/retest-1.13.0/lib/retest.rb:4: warning: observer was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add observer to your Gemfile or gemspec. Also contact author of retest-1.13.0 to add observer into its gemspec.

[AlexB52]

Thanks @AaronRustad, the change looks good.
I'll make a quick research on how this impacts previous ruby version from 2.5+ before we merge the change.
That said, it might all be fine.

[AlexB52]

Thank you @AaronRustad, it's been merged and released in 1.13.1
https://rubygems.org/gems/retest