Moved development dependencies to Gemfile

As expressions in a gemspec file are evaluated at the build time,
but not the run time, the conditional in the gemspec will not work
as intended.

Author: nobu

Gemfile

@@ -3,3 +3,11 @@ source "https://rubygems.org"
 git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
 
 gemspec
+
+group :development do
+  gem "bundler"
+  is_unix = RUBY_PLATFORM =~ /(aix|darwin|linux|(net|free|open)bsd|cygwin|solaris|irix|hpux)/i
+  gem 'vterm', '>= 0.0.5' if is_unix && ENV['WITH_VTERM']
+  gem 'yamatanooroti', '>= 0.0.6'
+  gem "rake"
+end

irb.gemspec

@@ -37,9 +37,4 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = Gem::Requirement.new(">= 2.5")
 
   spec.add_dependency "reline", ">= 0.1.5"
-  spec.add_development_dependency "bundler"
-  is_unix = RUBY_PLATFORM =~ /(aix|darwin|linux|(net|free|open)bsd|cygwin|solaris|irix|hpux)/i
-  spec.add_development_dependency 'vterm', '>= 0.0.5' if is_unix && ENV['WITH_VTERM']
-  spec.add_development_dependency 'yamatanooroti', '>= 0.0.6'
-  spec.add_development_dependency "rake"
 end