Removed http-request dependency.

Removes the http-request library dependency since it's only used [in one place](https://github.com/novoda/merlin/blob/master/core/src/main/java/com/novoda/merlin/service/HostPinger.java#L34) as mentioned in #20.

![sphagettti_scissors](https://cloud.githubusercontent.com/assets/1691740/4886498/d511bb38-637c-11e4-8d8c-2d3e64db3884.gif)