fix Illformed requirement error which causes with RubyGem 2.4.8

[Watson1978]

If RubyGem 2.4.8 was installed, rake spec causes error as following. This patch should fix error.

$ gem -v
2.4.8

$ rake spec
/Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/requirement.rb:100:in `parse': Illformed requirement [#<Gem::Requirement:0x007fd88aa56f30 @requirements=[["~>", #<Gem::Version "0.0">], [">=", #<Gem::Version "0.0.9">]]>] (Gem::Requirement::BadRequirementError)
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/requirement.rb:141:in `block in concat'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/requirement.rb:141:in `map'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/requirement.rb:141:in `concat'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/request_set.rb:117:in `gem'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/request_set/gem_dependency_api.rb:233:in `block in add_dependencies'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/request_set/gem_dependency_api.rb:232:in `each'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/request_set/gem_dependency_api.rb:232:in `add_dependencies'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/request_set/gem_dependency_api.rb:602:in `gemspec'
    from /Users/watson/tmp/cdq/Gemfile:3:in `load'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/request_set/gem_dependency_api.rb:278:in `instance_eval'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/request_set/gem_dependency_api.rb:278:in `load'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems/request_set.rb:289:in `load_gemdeps'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems.rb:1055:in `use_gemdeps'
    from /Users/watson/.rbenv/versions/2.2.2/lib/ruby/site_ruby/2.2.0/rubygems.rb:1244:in `<top (required)>'
    from <internal:gem_prelude>:1:in `require'
    from <internal:gem_prelude>:1:in `<compiled>'

[kemiller]

Whoa, thanks.