Bring project up to date

[bitwalker]

Hey @alexrp!

I was looking around for a parser combinator library, and was about to start writing my own when I stumbled across yours. It looks a little dusty, but nothing we can't fix :).

These three commits update the Elixir version requirement, update the dependencies to their latest versions, and updates the Travis CI configuration to use precompiled builds of Elixir and run on OTP 17.4.

I'll take a look through your issues list and see if there is anything else I can chew off as well.