Fix groovy dependency resolution error

This PR is a fix for #55. I simply upgraded Http-Builder from 0.5.2 to 0.7.1. The old version had a dependency on Groovy 1.5 or 1.7.99, neither of which appeared to be in MavenCentral anymore. 

The new version uses groovy 1.8.8, so I updated the gradle build script to also use 1.8.8 just to clean things up a bit. 

Tested on Entagen build servers