Ruby 2.x support

This PR adds Ruby 2.x support by upgrading `eventmachine` to 1.0.3.

All the tests pass (timing issue was causing one of the client specs to fail, which I've fixed), and it seems to work with `gnatsd`. Performance wise, running some of the benchmarks show that 2.1.2 is slightly faster but I haven't investigated much into performance yet.

Specs still pass on 1.9.3, didn't get around to checking other versions.