nth-prime: simplify example implementation

With exercism/exercism.io@54e1df3, we can now gather exercises into a
subdirectory. By separating the language track files from the
configuration and management files, it is easier to read the project.

- move gradle config under "exercises" too as it would make a mess
  otherwise.

Tests and example implementation based on the ruby exercise.

Completes a task for https://github.com/exercism/xjava/issues/3.