Ruby 3.1 no longer includes matrix in the stdlib

[rdlugosz]

As of Ruby 3.1, the matrix gem is no longer distributed as a part of the standard library and must be bundled explicitly.

I believe the fix is straightforward: include matrix in the gemspec file. However, this should probably be tested on a couple of different Ruby versions in order to verify the change doesn't cause problems for older Rubies.

In the meantime, users of the statsample gem can add matrix to their Gemfile and you'll satisfy the missing dependency. For example:

# https://github.com/SciRuby/statsample
gem "distribution"
gem "prime" # No longer in stdlib as of Ruby 3.1

gem "statsample"
gem "matrix" # No longer in stdlib as of Ruby 3.1

Note: similar issue opened on the distribution gem

[kojix2]

I think SciRuby needs more people with commit rights.