Loosen puma and rspec-rails version constraints in solidus_dev_support

Goal
----

As a solidus_starter_frontend user

I would like solidus_dev_support to support `puma ~> 5.0`

And I would like solidus_dev_support to support `rspec-rails ~> 5.0`

So that if solidus_starter_frontend is installed with specs on a Rails app with
those versions of the gems, solidus_starter_frontend won't have to revert the
gems to older version in order to support solidus_dev_support.

Background
----------

In
https://github.com/nebulab/solidus_starter_frontend/pull/175/files#r690067361,
`SolidusStarterFrontend::RspecGenerator#install` has to revert the puma version
of a Rails 6.1.4 app to 4.x. Otherwise, it won't be able to install
solidus_dev_support. Rails 6.1.4 sets puma to `'~> 5.0'`.

In solidusio/solidus_starter_frontend#180, the generator
also has to revert rspec-rails to `~> 4.0.0.beta3` in order to install
solidus_dev_support.

Author: gsmendoza

solidus_dev_support.gemspec

@@ -37,9 +37,9 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'ffaker', '~> 2.13'
   spec.add_dependency 'gem-release', '~> 2.1'
   spec.add_dependency 'github_changelog_generator', '~> 1.15'
-  spec.add_dependency 'puma', '~> 4.3'
+  spec.add_dependency 'puma', '>= 4.3', '< 6.0'
   spec.add_dependency 'rspec_junit_formatter'
-  spec.add_dependency 'rspec-rails', '~> 4.0.0.beta3'
+  spec.add_dependency 'rspec-rails', '>= 4.0.0.beta3', '< 6.0'
   spec.add_dependency 'rubocop', '~> 1.0'
   spec.add_dependency 'rubocop-performance', '~> 1.5'
   spec.add_dependency 'rubocop-rails', '~> 2.3'