Fix compatibility with beta versions of Sprockets

[patbl]

RubyGems made a change that causes this error when you're on a beta version of Sprockets 4:

/home/vagrant/code/rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/react-rails-2.6.0/lib/react/jsx/sprockets_strategy.rb:40:in
`register_engine_with_mime_type': undefined method `register_engine' for Sprockets:Module (NoMethodError)

Here's the relevant commit:

rubygems/rubygems@9836549#diff-94d386526b8b558035d4e4c7602d647c

And here's how the behavior of Gemf::Version#<=> has changed:

Before:

1:(main)> `gem --version`
=> "3.0.3\n"
2:(main)> Gem::Version.new("4.0.0.beta9") >= Gem::Version.new('4.x')
=> true
3:(main)> Gem::Version.new("4.0.0.beta9") >= Gem::Version.new('4.a')
=> true

After:

1:(main)> `gem --version`
=> "3.1.2\n"
2:(main)> Gem::Version.new("4.0.0.beta9") >= Gem::Version.new('4.x')
=> false
3:(main)> Gem::Version.new("4.0.0.beta9") >= Gem::Version.new('4.a')
=> true

Sprockets 4.0 is out now, but this is still a bug, and people shouldn't be forced to upgrade because of it.

[BookOfGreg]

Looks good, has tests, thank you for this gift :)