gems.rb: Conditionally install bake-modernize (not on JRuby).

[olleolleolle]

Description

CI fails, this PR attempts to make the build fail instead of error out. JRuby-related.

This fixes #45.

• JRuby: Avoid warning "Script coverage disabled: unknown event: script_compiled"
• JRuby: Enable ObjectSpace.
• JRuby: Skip a test to see if it was the only slow one. (It wasn't.)

Types of Changes

• Maintenance.

Testing

I ran the CI, and noted that it now runs-but-fails, as opposed to erroring out.

[olleolleolle]

Async::Container::Threaded
  behaves like Async::IO::SharedEndpoint
Error: The action has timed out.

This test consistently times out.

RSpec.describe Async::Container::Threaded, if: RUBY_PLATFORM !~ /darwin/ do
    it_behaves_like Async::IO::SharedEndpoint, described_class
end

Perhaps we can skip on JRuby, too?

Update: skipping on JRuby made another test be "where it times out":

Async::IO::Socket
  basic udp server
Error: The action has timed out.

[ioquatix]

Thanks.