Replaces old release tasks with new jCenter release script

Remove the old release tasks and replace them with the new ones in order to release on Bintray jCenter.
If we could replace these gist by a proper plugin and use a closure publish {} with the variables typed instead of environement one that would be a good + for the rest of the projects. The key and username could be defined in the gradle.properties on the CI and we would be more gradly and easier to evolve.