gradle: 2.14.1 -> 3.0, keep 2.14.1

[NeQuissimus]

Motivation for this change

Gradle 3.0 release (https://docs.gradle.org/3.0/release-notes)

I kept 2.14.1 since there are a number of removals and deprecations in 3.0, which may break Gradle for projects depending on old features.

I did, however, change the package versioning scheme to be more explicit (gradle_2_5 over gradle25)

Things done

• Tested using sandboxing
  (nix.useChroot on NixOS,
  or option build-use-chroot in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • OS X
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[mention-bot]

@NeQuissimus, thanks for your PR! By analyzing the annotation information on this pull request, we identified @volhovm, @reltuk and @MarcWeber to be potential reviewers

[NeQuissimus]

We are still ahead of the 16.09 cutoff, right? So this should make it into stable...

[NeQuissimus]

@joachifm @zimbatm @grahamc You guys are the last three to have merged Gradle-related PRs. Could we make sure to get this into 16.09? :) I would really appreciate this.

[NeQuissimus]

Thank you :)