Specified sbt version 0.12.4 for the purpose of building in Travis CI

It looks like Travis CI use sbt 0.13.0 now when it was specified without sbt version.
This is why fail build in Travis CI.
So, I specified sbt version using build.properties.

I wanted to set latest version 0.13.2, but I cannot find correspond spindle-codegen-plugin.
So, set 0.12.4.

I have confirmed the build success at my repository.

https://travis-ci.org/zaneli/rogue/builds/23980579
[![Build Status](https://travis-ci.org/zaneli/rogue.png?branch=sbt-0.12.4)](https://travis-ci.org/zaneli/rogue)