Add unix socket support

[sullerandras]

Continuation of #55
To test it locally i installed the new datadog agent to my Mac following the instructions here then used a simple ruby script like this:

require './lib/datadog/statsd'
require 'time'
STDOUT.sync = true

socket_file = ENV['DD_DOGSTATSD_SOCKET'] || '/tmp/socket/statsd.socket'
puts "========> socket_file: #{socket_file}"

client = Datadog::Statsd.new(nil, nil, socket_path: socket_file)

loop do
  client.increment('whatever')
  sleep(1)
end

I tried all scenarios listed in the testing guide and it worked.

[sullerandras]

Ruby 1.9 seems to fail because of this error.
Ruby 2.1 errors are strange. It works with older bundler <= 1.11.2 and seems to fail with any newer bundler.

[xvello]

hi @sullerandras , thanks a lot for your contribution!

I'm tagging the library maintainers to help figure out how to fix the tests.
@degemer @yannmh could you please have a look?

[xvello]

hi @sullerandras,

Could you please rebase on master, as tests are fixed now?

Regards

[masci]

@sullerandras tried to make tests on 2.1.10 pass with no success, do you have any idea what that could be? I think we can drop tests on jruby1.9 but 2.1 seems to work despite the tests :(

[sullerandras]

@masci It seems like MRI Ruby 2.1 comes with an embedded minitest 4.7.5 and rake uses that for running the specs. Minitest 5 is installed by bundler but it is not used.

[sullerandras]

@masci Changed to :direct loader and now it works with Ruby 2.1, but jruby still fails. Unfortunately i had to specify the spec file name so need to update this when adding more spec files.

[masci]

@sullerandras awesome, thanks! I think this is a good tradeoff for keeping 2.1 tests around.
Can you remove jruby from the CI? Not sure why we still keep it there....

[masci]

🎉