Update to Bukkit version to 1.7.10

Forge and Bukkit are now both on the same Minecraft version again.
Bukkit has not uploaded 1.7.10 to their repo, so use Spigot

It's only available on Github. Spigot has uploaded their 1.7.10 version
to their maven repo, so we can use that for now.
Fix Forge version not including common

Gradle files are read in the order they're written. Placing "include
project(":common")" above the dependency instruction will cause Shadow
to silently ignore it.
Make build file compatible with Gradle 2.0.

Had to update the Shadow version once more. Also added a few "project."
prefixes, as the scope rules changed. Still works on older gradle
versions.

Upgrading to Gradle 2.0 gives a nice speed boost: build times went from
37.3 to 28.3 seconds.