Supporting 1.9 in master

Hi!
We currently use a fork of ruby-smpp that is a merge of master and the 1.9 branch. (We have a couple small commits after this pull request)

I'd like to get back to using upstream/master, so I'd love to see 1.9 merged into master. That's what this pull request is asking, though I suspect this might not be the right place to ask that :-)

We use our fork (and therefore the 1.9 branch) on 1.8.7 in production, and I'm pretty confidant it's safe to merge for other 1.8 people.

Let me know if there's a better way to go about this!