fix active_support dependency in gemspec

I got this error. Looks like the activesupport gem is not included as a runtime dependency.

/usr/local/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require': cannot load such file -- active_support/core_ext/numeric (LoadError)
        from /usr/local/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require'
        from /usr/local/lib/ruby/gems/2.0.0/gems/docker-client-0.1.4/lib/docker.rb:1:in `<top (required)>'