Support Ruby 2.3.0

[tricknotes]

In Ruby 2.3.0, UDPSocket#recvfrom's flags doesn't allow nil.
It should be Numeric value.

So I specified this default value to the same value as UDPSocket#recvfrom's.

[tricknotes]

Currently many specs are failed.
However I wish #162 will fix them.

[tarcieri]

Looks benign enough despite the existing test failures

[tricknotes]

Oops, I missed some failing specs such as the following:

  1) Celluloid::IO::DNSResolver#resolve resolves hostnames
     Failure/Error: expect(resolver.resolve("localhost")).to eq(Resolv::IPv4.create("127.0.0.1")).or eq(Resolv::IPv6.create("::1"))

     RuntimeError:
       can't modify frozen String
     # /home/travis/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/resolv.rb:2463:in `block in create'
     # /home/travis/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/resolv.rb:2463:in `scan'
     # /home/travis/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/resolv.rb:2463:in `create'
     # ./spec/celluloid/io/dns_resolver_spec.rb:15:in `block (3 levels) in <top (required)>'

https://travis-ci.org/celluloid/celluloid-io/jobs/99525963

However this is a Ruby's bug: https://bugs.ruby-lang.org/issues/11910
And it is fixed on trunk.

Should we remove 2.3.0 from .travis.yml until Ruby 2.3.1 released?

[tricknotes]

Thanks for your merge ✨
I'm happy if you release a new version with this change ❤️

[tarcieri]

@digitalextremist were you planning a release for this soon? Otherwise I can. It seems like all of the projects could use another release.

[ioquatix]

Yes I'd like to see another release with the changes including the socket base class. Can we please get the rspec/testing issues fixed and then I will merge that code then we do a release? Perhaps 0.17.3? I'd also like to have a hangout with everyone at some point to discuss the path to 1.0.0

[dblock]

+1 on a release, want for https://github.com/dblock/slack-ruby-client

[tarcieri]

Okay, I'll do a release this weekend