Fix version check load order issue

[Earlopain]

Checking the version causes an error when simplecov-html is loaded before simplecov:

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "simplecov-html"
  gem "simplecov"
end

/home/user/code/simplecov-html/lib/simplecov-html.rb:10:in '<top (required)>': uninitialized constant SimpleCov::VERSION (NameError)

major, minor, patch = SimpleCov::VERSION.scan(/\d+/).first(3).map(&:to_i)
                               ^^^^^^^^^
        from <internal:/home/user/.rbenv/versions/3.4-dev/lib/ruby/3.4.0+0/rubygems/core_ext/kernel_require.rb>:37:in 'Kernel#require'

I don't think this check is even needed anymore but I'd just leave it in

[Earlopain]

Hi @amatsuda, sorry to be a bother. Could you consider merging this? I ran into this again and would appreciate you taking a look. Thank you.