Relax Bundler version for development (#1086)

The upper bound on the Bundler version introduced in
9c34ef3 is causing
installation errors with the development version of Ruby 4.1-dev.

```console
> bundle install
(snip)

Could not find compatible versions

Because the current Bundler version (4.1.0.dev) does not satisfy bundler >=
1.15, < 3.0.0
and Gemfile depends on bundler >= 1.15, < 3.0.0,
  version solving has failed.
```

https://github.com/whitequark/parser/actions/runs/20980305950/job/60303672289

In practice, there is little need to specify an upper bound
unless an actual compatibility issue with the Bundler version used for development arises.

Author: koic

parser.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency             'ast',       '~> 2.4.1'
   spec.add_dependency             'racc'
 
-  spec.add_development_dependency 'bundler',   '>= 1.15', '< 3.0.0'
+  spec.add_development_dependency 'bundler',   '>= 1.15'
   spec.add_development_dependency 'rake',      '~> 13.0.1'
   spec.add_development_dependency 'cliver',    '~> 0.3.2'