gemspec: Fix version syntax.

Author: esteban zapata

ruby-statistics.gemspec

@@ -27,9 +27,9 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_development_dependency "bundler", '>= 1.15.4'
-  spec.add_development_dependency "rake", '>= 12.0.0'
-  spec.add_development_dependency "rspec", '>= 3.6.0'
-  spec.add_development_dependency "grb", '0.4.1'
-  spec.add_development_dependency 'byebug', '9.1.0'
+  spec.add_development_dependency "bundler",'~> 1.15', '>= 1.15.4'
+  spec.add_development_dependency "rake", '~> 12.0', '>= 12.0.0'
+  spec.add_development_dependency "rspec", '~> 3.6', '>= 3.6.0'
+  spec.add_development_dependency "grb", '~> 0.4.1', '>= 0.4.1'
+  spec.add_development_dependency 'byebug', '~> 9.1.0', '>= 9.1.0'
 end