--- a/rmagick.gemspec
+++ b/rmagick.gemspec
@@ -2,13 +2,6 @@
 require 'English'
 require './lib/rmagick/version'
 
-def v(version)
-  Gem::Version.new(version)
-end
-
-RUBY     = v(RUBY_VERSION.dup)
-RUBYGEMS = v(Gem::VERSION.dup)
-
 Gem::Specification.new do |s|
   s.name = 'rmagick'
   s.version = Magick::VERSION
@@ -20,7 +13,7 @@
   s.homepage = 'https://github.com/rmagick/rmagick'
   s.license = 'MIT'
 
-  tracked_files = `git ls-files`.split($OUTPUT_RECORD_SEPARATOR)
+  tracked_files = Dir['deprecated/*', 'ext/**/*', 'lib/**/*', '*.gemspec']
   file_exclusion_regex = %r{(\Alib/rvg/to_c.rb)}
   files         = tracked_files.reject{|file| file[file_exclusion_regex] }
   test_files    = files.grep(%r{^(test|spec|features)/})
@@ -38,26 +31,6 @@
   s.requirements << "ImageMagick #{Magick::MIN_IM_VERSION} or later"
 
   s.add_development_dependency 'rspec', '~> 3.2.0'
-
-  if RUBY < v('1.9.0')
-    s.add_development_dependency 'rake', '~> 10.0'
-  end
-
-  if RUBY < v('2.0.0')
-    s.add_development_dependency 'json', '~> 1.0'
-  end
-
-  if RUBY >= v('1.9.2')
-    s.add_development_dependency 'rubocop', '~> 0.33.0'
-  end
-
-  if RUBY >= v('2.2.0')
-    s.add_development_dependency 'test-unit', '~> 2'
-  end
-
-  if RUBYGEMS < v('1.8.25')
-    s.add_development_dependency 'rake-compiler', '~> 0.8.0'
-  else
-    s.add_development_dependency 'rake-compiler'
-  end
+  s.add_development_dependency 'test-unit', '~> 2'
+  s.add_development_dependency 'rake-compiler'
 end