Allow simplecov 1.x while keeping Ruby 3.1/3.2 installable

simplecov 1.3.0 requires Ruby >= 3.3, so pinning `~> 1.3` (as proposed
in #764) breaks bundle resolution on Ruby 3.1 and 3.2, which split still
supports. Widen the requirement to `>= 0.15, < 2` so Bundler picks the
newest simplecov compatible with each Ruby (0.22 on 3.1, 1.2 on 3.2,
1.3 on 3.3+).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: snaka

split.gemspec

@@ -39,7 +39,7 @@ Gem::Specification.new do |s|
   s.add_dependency "cgi"
 
   s.add_development_dependency "bundler",     ">= 1.17"
-  s.add_development_dependency "simplecov",   "~> 0.15"
+  s.add_development_dependency "simplecov",   ">= 0.15", "< 2"
   s.add_development_dependency "rack-test",   "~> 2.0"
   s.add_development_dependency "rake",        "~> 13"
   s.add_development_dependency "rspec",       "~> 3.7"