Got rid of nexus plugin for compatiblity with Gradle 2.x

All the setup is working with CI on Travis, snapshots in oss.jfrog.org and releases on Bintray:

https://travis-ci.org/jbaruch/gradle-fatjar-plugin/builds
https://oss.jfrog.org/webapp/builds/gradle-fatjar-plugin
https://bintray.com/jbaruch/jbaruch-maven/eu.appsatori%3Agradle-fatjar-plugin/view

I'll be happy to migrate the package in Bintray to you personally or to an organization once created.