Remove conditional on bigdecimal

- bigdecimal installs and works fine on all supported ruby versions
- The condition was checking for ruby version to be greater than 6.19,
  which will not happen soon
- The condition is also evaluated on `rubygems`, and it is not correct

Close #1085

money.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |s|
   s.license     = "MIT"
 
 
-  s.add_dependency "bigdecimal", "~> 3.1" if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('6.19.0')
+  s.add_dependency "bigdecimal"
   s.add_dependency 'i18n', [">= 0.6.4", '<= 2']
 
   s.add_development_dependency "bundler"